PR #15417: Add while loop config options and optional pass pipeline immediately before unroll.

[copybara-service[bot]]

PR #15417: Add while loop config options and optional pass pipeline immediately before unroll.

Imported from GitHub PR https://github.com/openxla/xla/pull/15417

This PR adds the availability to configure while loop unroll thresholds. Existing defaults are maintained. This PR also adds the option for the user to specify an HloPassPipeline that will be run immediately before unroll. This is very important and by design. Some proprietary passes need to be run immediately before the unroll as the last step. The pipeline is defaulted to nullptr. Copybara import of the project:

-- 3960c3aa512e9d118c8e4c16679806a46b985cf0 by ptoulme-aws [email protected]:

Add while loop config options and optional pass pipeline immediately before unroll

Merging this change closes #15417

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/15417 from ptoulme-aws:while_loop_enhancement 3960c3aa512e9d118c8e4c16679806a46b985cf0