[TF:XLA] Consolidate requires_compilation and enable_jit_by_default into a new...

[TF:XLA] Consolidate requires_compilation and enable_jit_by_default into a new autoclustering_policy field.

The concept of "requires_compilation" has been fairly confusing ever since ondemand mode (aka. op-by-op compilation) has been checked in. Compilation here actually meant "autoclustering" and no device actually requires it any more.

Don't form clusters of size 1 if autoclustering on what was formerly a "requires_compilation" device; the on-demand code path achieves the same goal.

We can also move the policy decision about whether to jit on the CPU device into the autoclustering policy field, which is simpler.

PiperOrigin-RevId: 220152352