[DDP] Param to name mapping in Reducer
Constructs and passes in a mapping with parameter names to Reducer to log information about unused parameters in error messages about unused parameters/not all parameters getting gradient.
Use case:
1) User runs DDP forward + bwd, and it has some unused parameters that will result in ddp error in next iteration
2) Next forward pass calls `Reducer::ensure_prior_reduction_finished()` where we check all params got gradient from the previous bwd pass. DDP would throw here in this case.
3) Reducer maintains mapping and tracks used parameters, and computes which parameters did not get gradient and logs this as part of the error.
Implementation details:
0) The following is only enabled for debug modes of INFO or DETAIL.
1) To save memory, we don't map param -> param name so that we don't have to copy the entire tensor, instead we map param_index -> param_name and use the existing concept of variable_index in Reducer to look up parameter names.
2) DDP constructs param index -> param name mapping. The name is the fully qualified name: f"{module_name}:{param_name}" and passes it into Reducer
3) Reducer maintains per-iteration std::set<int> of variable indices that have had `mark_variable_ready` called.
4) When some params go unused, we take a set difference to detect the unused params.
5) Unittests to test the logged unused params, as well as for nested modules, are added
Differential Revision: [D27356394](https://our.internmc.facebook.com/intern/diff/D27356394/)
[ghstack-poisoned]
