Fix pipe tests failures on CPU

[jinyouzhi]

Follow up #8381 @delock

fix

unit/pipe/test_pipe_module.py
unit/runtime/pipe/test_pipe.py

This pull request refactors device handling in the activation checkpointing code to simplify and standardize how device variables are used. The main change is to consistently use a device variable instead of cuda_device and to remove unused or redundant stream-related code. This improves code clarity and maintainability without changing functionality.

Device handling simplification:

• Replaced all occurrences of cuda_device with a unified device variable obtained from get_accelerator().current_device_name() in checkpointing.py. [1] [2] [3]
• Updated all device argument usages in calls to copy_to_device, gather_partitioned_activations, and move_to_device to use the new device variable. [1] [2] [3] [4]

Stream and code cleanup:

• Removed creation and usage of the transport_stream variable, and deleted commented-out or unused stream synchronization code.

These changes make the device management more straightforward and reduce potential confusion around device and stream handling in activation checkpointing.