[TPU] make ptxla not imported when using tpu_commons

[yaochengji]

Purpose

make ptxla not imported when using tpu_commons

Test Plan

All the current CI tests

Test Result

Passed

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request refactors the codebase to conditionally import torch_xla and its related modules only when the tpu_commons library is not in use. This is achieved by introducing a USE_TPU_COMMONS flag, which is a good approach for managing optional dependencies. The changes are applied across several TPU-related files, making the imports lazy and conditional. My main feedback concerns an inconsistency in vllm/v1/attention/backends/pallas.py, where the check for tpu_commons differs from other files, potentially leading to robustness issues.

[vanbasten23]

Could you summarize the problem that this PR tries to address?

[yaochengji]

Could you summarize the problem that this PR tries to address?

@vanbasten23 by cleaning up the import logic, it prevents tpu_commons from failing due to ptxla

[DarkLight1337]

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