[Core] Cache the expensive TPUAcceleratorManager.get_current_node_num_accelerators

[jjyao]

Why are these changes needed?

TPUAcceleratorManager.get_current_node_num_accelerators is expensive and it doesn't change during the lifetime of the program so we can cache it.

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[jjyao]

Perf is good: https://buildkite.com/ray-project/release/builds/17756#0190283a-0175-474b-9452-414a8049d4e9