fix(comm): Expose GradBucket in deepspeed.comm API (#7400)

This PR fixes an omission in the `deepspeed.comm` API where `GradBucket`
was not exposed, despite the package aiming for full compatibility with
`torch.distributed`.

##The Problem

As reported in issue #7393, when a user replaces `torch.distributed`
with `deepspeed.comm`, they expect all public APIs to be available.
However, attempting to access `deepspeed.comm.GradBucket` (for example,
when using it as a type hint for DDP communication hooks) results in an
`AttributeError`.

##The Solution

This change resolves the issue by importing `GradBucket` directly from
`torch.distributed` into the `deepspeed/comm/comm.py` file, making it
part of the public `deepspeed.comm` namespace.

A `# noqa: F401` comment has been added to the import line. This is
necessary to bypass the `flake8` linter's "imported but unused" check,
as the specific purpose of this import is to expose the symbol to the
end-user, not for it to be used within the `comm.py` file itself.

##How This Was Tested

The fix was verified with a local test script that confirms
`deepspeed.comm.GradBucket` can now be accessed correctly and is
identical to `torch.distributed.GradBucket`. The pre-commit hooks now
pass successfully.

##Related run test Screenshout 

<img width="1250" alt="Screenshot 2025-06-30 at 22 41 10"
src="https://github.com/user-attachments/assets/cadf18e1-9d1a-4164-a5ff-0b3e6804ac48"
/>

##Related Issue
Fixes #7393

Signed-off-by: Vensenmu <[email protected]>
Co-authored-by: Masahiro Tanaka <[email protected]>

Author: Flink-ddd

deepspeed/comm/comm.py

@@ -22,6 +22,7 @@
 """
 
 import torch
+from torch.distributed import GradBucket  # noqa: F401
 import os
 
 from ..constants import TORCH_DISTRIBUTED_DEFAULT_PORT, default_pg_timeout