[xla] Extend collective-pipeliner to support loop variant parameters in
[xla] Extend collective-pipeliner to support loop variant parameters in
backward chains.

Add a predicate to the collective-pipeliner config for a backend to specify
whether an individual loop variant parameter is allowed in a pipelining chain.
This predicate is currently only used to support kBackward pipelining.

Enhance TransformLoopBackward to handle loop variant parameter in backward
chains.

Add a test.
