🚀 Feature

Move allgather_coalesced functionality to comm.cpp.

Motivation

allgather_coalesced does not rely on any backend specific functionality as it just uses process group's allgather. Therefore, by moving it to some more generic place we can get all supported usecases of allgather to be supported for allgather_coalesced

Additional context

This was requested as a part of review of PR 28857 #28857

cc @pietern @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @xush6528