Update branch references -> main (#577)

Author: texasmichelle

Diff for: BRANCHES.md

@@ -1,5 +1,5 @@
-Notebooks on the `master` branch should be compatible with the toolchain that is
+Notebooks on the `main` branch should be compatible with the toolchain that is
 released in Colab.
 
 Make incompatible changes to notebooks in the `nightly-notebooks` branch. We
-promote `nightly-notebooks` to `master` when we deploy new toolchains to Colab.
+promote `nightly-notebooks` to `main` when we deploy new toolchains to Colab.

Diff for: Contributing.md

@@ -2,7 +2,7 @@
 
 Everyone is welcome to contribute to Swift for TensorFlow. Contributing doesn't just mean submitting code - there are many different ways to get involved, including reporting bugs, improving documentation, writing models or tutorials, submitting deep learning building blocks, or participating in API and infrastructure design discussions.
 
-The Swift for TensorFlow community is guided by our [Code of Conduct](https://github.com/tensorflow/swift/blob/master/CODE_OF_CONDUCT.md), which we encourage everybody to read before participating.
+The Swift for TensorFlow community is guided by our [Code of Conduct](https://github.com/tensorflow/swift/blob/main/CODE_OF_CONDUCT.md), which we encourage everybody to read before participating.
 
 ## Report bugs
 
@@ -35,7 +35,7 @@ Follow the [code contribution workflow](#code-contribution-workflow) when contri
 
 We discuss preliminary feature requests and ideas on the [mailing list](https://groups.google.com/a/tensorflow.org/forum/#!forum/swift). You can participate by sending your own feature requests and ideas to the mailing list, or by commenting on others' feature requests and ideas.
 
-Once an idea has been fleshed out, the person or people driving it write a proposal and send it as a PR to the [proposals directory](https://github.com/tensorflow/swift/tree/master/proposals). Further discussion happens on that PR, and the PR gets merged if the design gets accepted. You can participate by proposing your own proposals, or by commenting on others' proposals.
+Once an idea has been fleshed out, the person or people driving it write a proposal and send it as a PR to the [proposals directory](https://github.com/tensorflow/swift/tree/main/proposals). Further discussion happens on that PR, and the PR gets merged if the design gets accepted. You can participate by proposing your own proposals, or by commenting on others' proposals.
 
 
 ## Code contribution workflow

Diff for: FAQ.md

@@ -6,7 +6,7 @@ encountered and prefer not to get new issues filed in our bug tracker.
 
 * [Why Swift?](#why-swift)
 * [How can I use Python 3 with the Python module?](#how-can-i-use-python-3-with-the-python-module)
-* [\[Mac\] I wrote some code in an Xcode Playground. Why is it frozen/hanging?](https://github.com/tensorflow/swift/blob/master/FAQ.md#mac-i-wrote-some-code-in-an-xcode-playground-why-is-it-frozenhanging)
+* [\[Mac\] I wrote some code in an Xcode Playground. Why is it frozen/hanging?](https://github.com/tensorflow/swift/blob/main/FAQ.md#mac-i-wrote-some-code-in-an-xcode-playground-why-is-it-frozenhanging)
 
 ## Why Swift?
 

Diff for: README.md

@@ -53,14 +53,14 @@ The Swift for TensorFlow project is currently focusing on 2 kinds of users:
 
 Tutorial | Last Updated |
 -------- | ------------ |
-[A Swift Tour](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/a_swift_tour.ipynb) | March 2019
-[Protocol-Oriented Programming & Generics](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/protocol_oriented_generics.ipynb) | August 2019
-[Python Interoperability](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/python_interoperability.ipynb) | March 2019
-[Custom Differentiation](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/custom_differentiation.ipynb) | March 2019
-[Sharp Edges in Differentiability](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/Swift_autodiff_sharp_edges.ipynb) | November 2020
-[Model Training Walkthrough](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/model_training_walkthrough.ipynb) | March 2019
-[Raw TensorFlow Operators](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/raw_tensorflow_operators.ipynb) | December 2019
-[Introducing X10, an XLA-Based Backend](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/introducing_x10.ipynb) | May 2020
+[A Swift Tour](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/a_swift_tour.ipynb) | March 2019
+[Protocol-Oriented Programming & Generics](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/protocol_oriented_generics.ipynb) | August 2019
+[Python Interoperability](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/python_interoperability.ipynb) | March 2019
+[Custom Differentiation](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/custom_differentiation.ipynb) | March 2019
+[Sharp Edges in Differentiability](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/Swift_autodiff_sharp_edges.ipynb) | November 2020
+[Model Training Walkthrough](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/model_training_walkthrough.ipynb) | March 2019
+[Raw TensorFlow Operators](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/raw_tensorflow_operators.ipynb) | December 2019
+[Introducing X10, an XLA-Based Backend](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/introducing_x10.ipynb) | May 2020
 
 ### Resources
 
@@ -122,7 +122,7 @@ to make users more productive. You can run Swift interactively in a Jupyter
 notebook, and get helpful autocomplete suggestions to help you explore the
 massive API surface of a modern deep learning library. You can [get started
 right in your browser in
-seconds](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/model_training_walkthrough.ipynb)!
+seconds](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/model_training_walkthrough.ipynb)!
 
 Migrating to Swift for TensorFlow is really easy thanks to Swift's powerful
 Python integration. You can incrementally migrate your Python code over (or
@@ -160,7 +160,7 @@ out the following documentation.
 
 Document | Last Updated | Status |
 -------- | ------------ | ------ |
-[Swift Differentiable Programming Manifesto](https://github.com/apple/swift/blob/master/docs/DifferentiableProgramming.md) | January 2020 | Current
+[Swift Differentiable Programming Manifesto](https://github.com/apple/swift/blob/main/docs/DifferentiableProgramming.md) | January 2020 | Current
 [Swift Differentiable Programming Implementation Overview](https://docs.google.com/document/d/1_BirmTqdotglwNTOcYAW-ib6mx_jl-gH9Dbg4WmHZh0) | August 2019 | Current
 [Swift Differentiable Programming Design Overview](https://docs.google.com/document/d/1bPepWLfRQa6CtXqKA8CDQ87uZHixNav-TFjLSisuKag/edit?usp=sharing) | June 2019 | Outdated
 [Differentiable Types](docs/DifferentiableTypes.md) | March 2019 | Outdated

Diff for: RELEASES.md

@@ -52,7 +52,7 @@ and refining APIs.
   `Layer`s are now "callable" like functions, e.g. `layer(input)`.
   * **Note: this is experimental functionality that is currently [being proposed
     through Swift
-    Evolution](https://github.com/apple/swift-evolution/blob/master/proposals/0253-callable.md).**
+    Evolution](https://github.com/apple/swift-evolution/blob/main/proposals/0253-callable.md).**
     Expect potential changes.
 * The `context` argument has been removed from `Layer`'s `applied(to:)` method.
   Instead, contexts are now thread-local. ([swift-apis#87](https://github.com/tensorflow/swift-apis/pull/87))
@@ -125,7 +125,7 @@ natural Python interoperability.
 
 * Hit "Tab" to trigger basic semantic autocomplete.
 * [Use matplotlib to produce inline
-  graphs.](https://github.com/google/swift-jupyter/blob/master/README.md#rich-output)
+  graphs.](https://github.com/google/swift-jupyter/blob/main/README.md#rich-output)
 * Interrupt cell execution by clicking the "stop" button next to the cell.
 
 ### Swift Standard Library Enhancements
@@ -134,7 +134,7 @@ natural Python interoperability.
   [`KeyPathIterable`](https://www.tensorflow.org/swift/api_docs/Protocols/KeyPathIterable)
   protocol conformance to make your custom type provide a collection of key
   paths to stored properties. Read [Dynamic Property Iteration using Key
-  Paths](https://github.com/tensorflow/swift/blob/master/docs/DynamicPropertyIteration.md)
+  Paths](https://github.com/tensorflow/swift/blob/main/docs/DynamicPropertyIteration.md)
   for a deep dive into the design.
 * Declare an
   [`AdditiveArithmetic`](https://www.tensorflow.org/swift/api_docs/Protocols/KeyPathIterable)
@@ -176,13 +176,13 @@ natural Python interoperability.
   [`Differentiable`](https://www.tensorflow.org/swift/api_docs/Protocols/Differentiable)
   protocol to make a custom type work with automatic differentiation. For a
   technical deep dive, read [Differentiable
-  Types](https://github.com/tensorflow/swift/blob/master/docs/DifferentiableTypes.md).
+  Types](https://github.com/tensorflow/swift/blob/main/docs/DifferentiableTypes.md).
 * Use
   [`differentiableFunction(from:)`](https://www.tensorflow.org/swift/api_docs/Functions#/s:10TensorFlow22differentiableFunction4fromq0_x_q_tcq0_5value_15CotangentVectorQz_AEQy_tAEQy0_c8pullbacktx_q_tc_tAA14DifferentiableRzAaJR_AaJR0_r1_lF)
   to form a `@differentiable` function from a custom derivative function.
 * Custom differentiation APIs are available in the standard library. Follow the
   [custom differentiation
-  tutorial](https://colab.research.google.com/github/tensorflow/swift/blob/master/docs/site/tutorials/custom_differentiation.ipynb)
+  tutorial](https://colab.research.google.com/github/tensorflow/swift/blob/main/docs/site/tutorials/custom_differentiation.ipynb)
   to learn how to use them.
   * Gradient checkpointing API:
     [`withRecomputationInPullbacks(_:)`](https://www.tensorflow.org/swift/api_docs/Protocols/Differentiable#/s:10TensorFlow14DifferentiablePAAE28withRecomputationInPullbacksyqd__qd__xcAaBRd__lF).