[Fault Tolerance] Don't check the len of a dataset, but its instance. (#10432)

Co-authored-by: Thomas Chaton <[email protected]>

Author: awaelchli

CHANGELOG.md

@@ -17,6 +17,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed the logging with `on_step=True` in epoch-level hooks causing unintended side-effects. Logging with `on_step=True` in epoch-level hooks will now correctly raise an error ([#10409](https://github.com/PyTorchLightning/pytorch-lightning/pull/10409))
 - Fixed deadlocks for distributed training with `RichProgressBar` ([#10428](https://github.com/PyTorchLightning/pytorch-lightning/pull/10428))
 - Fixed an issue where the model wrapper in Lite converted non-floating point tensors to float ([#10429](https://github.com/PyTorchLightning/pytorch-lightning/pull/10429))
+- Fixed an issue with inferring the dataset type in fault-tolerant training ([#10432](https://github.com/PyTorchLightning/pytorch-lightning/pull/10432))
 
 
 ## [1.5.0] - 2021-11-02

pytorch_lightning/trainer/data_loading.py

@@ -37,7 +37,7 @@
     CaptureMapDataset,
     FastForwardSampler,
 )
-from pytorch_lightning.utilities.data import has_iterable_dataset, has_len_all_ranks
+from pytorch_lightning.utilities.data import get_len, has_iterable_dataset, has_len_all_ranks
 from pytorch_lightning.utilities.enums import DistributedType
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
 from pytorch_lightning.utilities.imports import _fault_tolerant_training
@@ -282,10 +282,11 @@ def _get_dataloader_init_kwargs(
             dl_kwargs["sampler"] = None
 
         if _fault_tolerant_training():
-            if isinstance(dl_kwargs["dataset"], IterableDataset):
+            dataset = dl_kwargs["dataset"]
+            if isinstance(dataset, IterableDataset):
                 # wrap the `IterableDataset` into a `CaptureIterableDataset` to record sampler states.
                 dl_kwargs["dataset"] = CaptureIterableDataset(dataset=dl_kwargs["dataset"])
-            elif len(dl_kwargs["dataset"]):
+            elif get_len(dataset) != float("inf"):
                 dl_kwargs["dataset"] = CaptureMapDataset(dataset=dl_kwargs["dataset"])
             else:
                 raise MisconfigurationException(