Currently LayoutAssignment only deals with a module at a time and

knows nothing about other modules. Therefore, sends and recvs are
given a default layout to ensure that the layout is the same across
all communicating modules.

This CL adds a ChannelLayoutConstraints object which can be passed
into LayoutAssignment. When a send or recv is layed out, its chosen
layout is saved into the ChannelLayoutConstraints. When, later,
another instruction is seen in a different module that uses the same
communicating channel ID, the layout is constrained to be the same as
in the previous module.

PiperOrigin-RevId: 180771799