[TPU] support disabling xla compilation cache

[yaochengji]

The PyTorch/XLA compilation cache uses the Torch IR to generate keys. Consequently, changes in optimization flags, which affect compilation results, don't change the cache key. This can result in the wrong compilation being used. To prevent this, disabling the XLA compilation cache during development is recommended. We can disable it by export VLLM_XLA_CACHE_PATH=

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[alexm-redhat]

Good catch @yaochengji !