[XPU] Add an implict conversion from XPUStream to sycl::queue* (pytorch#148646)

# Motivation

Currently, in Pytorch XPU, `cudaStream_t` is mapped to `sycl::queue&`, so an implicit cast from `XPUStream` to `sycl::queue&` is provided just like `CUDAStream` has an implicit cast to `cudaStream_t`.

But on the SYCLomatic side, we migrate `cudaStream_t` to `sycl::queue*` but not `sycl::queue&` (One reason is that `cudaStream_t` is actually a pointer so users can do anything with that integer. Another reason is that the early `sycl::queue` was not impl-ed by a pointer, so copy by value is not desirable.)

Without this PR:
```
cudaStream_t a = getCurrentCUDAStream();
cudaStream_t b = getCurrentCUDAStream().stream();
```
need be migrated to:
```
queue_ptr a = &(sycl::queue&)getCurrentXPUStream();
queue_ptr b = &(getCurrentXPUStream().queue());
```
With this PR:
```
queue_ptr a = getCurrentXPUStream();
queue_ptr b = &(getCurrentXPUStream().queue());
```
Pull Request resolved: pytorch#148646
Approved by: https://github.com/guangyey, https://github.com/EikanWang

Author: zhiweij1

c10/xpu/XPUStream.h

@@ -59,6 +59,11 @@ class C10_XPU_API XPUStream {
     return queue();
   }
 
+  /// Implicit conversion to sycl::queue*.
+  operator sycl::queue*() const {
+    return &queue();
+  }
+
   /// Implicit conversion to Stream (a.k.a., forget that the stream is a
   /// XPU stream).
   operator Stream() const {

c10/xpu/test/impl/XPUStreamTest.cpp

@@ -223,6 +223,9 @@ TEST(XPUStreamTest, ExternalTest) {
   ASSERT_TRUE(curStream == myStream);
   ASSERT_TRUE(&(curStream.queue()) == stream);
 
+  sycl::queue* q_ptr = curStream;
+  ASSERT_TRUE(q_ptr == stream);
+
   delete stream;
 }