Allow for custom sharding specs to register their own ops.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76360
Customized ShardingSpecs could be entirely arbitrary and it would not
be possible to handle ops for those as a result since they might not fit into
the patterns supported by the in-built ShardingSpecs. As a result, we introduce
a framework for a ShardingSpec to override ops as follows:
1) In the dispatch system, if a ShardingSpec has a customized op the registered
op for that ShardingSpec is invoked.
2) As a result, all ChunkShardingSpec specific ops have been moved under that
ShardingSpec.
3) There will be a set of ShardingSpec agnostic ops (ex: elementwise ops) which
will be a set of common ops supported across any ShardingSpec.
4) If an op is not found for a particular ShardingSpec the default set of ops
is searched for that op.
Differential Revision: [D35917912](https://our.internmc.facebook.com/intern/diff/D35917912/)
Approved by: https://github.com/wanchaol
