[XLA] Add some plumbing, documentation, verification and shape inference for Gather

Pretty much everything other than HLO verification and shape inference will fail
for Gather with Unimplemented.

Note that this CL is intentionally incomplete -- I figured it would be nicer to
get some of the boiler-platey stuff out of the way early.  Let me know if you
want me to send in a larger but more complete CL instead.

PiperOrigin-RevId: 186055521