Use caller CUDA stream for D2H and H2D copies (#20498)

[Conarnar]

Summary:

CudaAllocator memory copies now support async copy on a caller-provided CUDA stream. When a caller stream is available (via getCallerStream()), copy_host_to_device and copy_device_to_host use cudaMemcpyAsync and synchronize the stream before returning — preserving the blocking API contract while allowing work to be issued on the caller's stream. When no caller stream is set, the synchronous cudaMemcpy path is used as before.

Additionally:

• Added null pointer and zero-byte validation — null dst/src return Error::InvalidArgument instead of aborting in cudaMemcpy, and zero-byte copies return Error::Ok early.
• Assert single-GPU case (index 0 or -1) until multi-GPU stream validation is added.
• Wired //executorch/extension/cuda:caller_stream dependency in TARGETS.
• Added test_cuda_allocator with coverage for sync/async paths and error handling.

Differential Revision: D109590531

[pytorch-bot]

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❗ 2 Active SEVs

There are 2 currently active SEVs. If your PR is affected, please view them below:

• [ROCm] MI350 CI runner label rename: rebase PRs using old linux.rocm.gpu.gfx950.* labels
• [Housekeeping] Rename uw1 cluster to meta-prod-aws-uw1

❌ 3 New Failures, 1 Unrelated Failure

As of commit 3d8da75 with merge base 45a14b9 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
  >>> Lint for backends/cuda/CMakeLists.txt:
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 53fd3bdbd1cb742e07ca658dafb16785263edd8aaeaccd6563651265393f6324 /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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