[XLA:TPU] Fix a bug in GetGatherScatterBatchParallelDims.
[XLA:TPU] Fix a bug in GetGatherScatterBatchParallelDims.

The bug is that the function does not handle the case where the indices are a copy of a concatenate. Thus this we add code to search up a tree of copies to see if the first non-copy op is a concatenate, in which case we can use that to test if it is a concatenate of an iota, to then find parallel dims.
