Update XLA support

[bfontain]

What does this PR do?

This change contains a few small updates to XLA support in composer:

1. Updates initialization to add required init_method argument for xla.
2. Inserts a 'xm.mark_step()' call in the microbatching loop to prevent unrolling of microbatching.

What issue(s) does this change relate to?

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