[c10d/nccl-pg] allow user to pass process group description (#123472)

Summary:
We need a way to allow user set a customized description for a process group, e.g. FSDP, PP.

Here are several use cases of user specified group_desc:
- Logging: we can easily match a log line and understand what's this collective/pg is used to.
- Pytorch traces (e.g. Kineto, Execution Trace) can benefit from the PG desc since trace analysis, benchmarks will be able to easily differentiate PG purpose like FSDP, PP.
- Lower layer collectives(e.g. NCCL) debug: we will be able to expose PG desc to NCCL communicator so NCCL layer operations can be easily correlated to a PG.

Solution: Add a group_desc field to c10d

Differential Revision: D55781850

Pull Request resolved: https://github.com/pytorch/pytorch/pull/123472
Approved by: https://github.com/kwen2501

torch/csrc/distributed/c10d/ProcessGroup.hpp

@@ -694,6 +694,8 @@ class TORCH_API ProcessGroup : public torch::CustomClassHolder {
 
   const std::string& getGroupName() const;
   void setGroupName(const std::string& name);
+  const std::string& getGroupDesc() const;
+  void setGroupDesc(const std::string& name);
   void enableCollectivesTiming();
 
   void release_resources() override;
@@ -724,6 +726,7 @@ class TORCH_API ProcessGroup : public torch::CustomClassHolder {
   const int size_;
   const c10::intrusive_ptr<Options> options_;
   const BackendType backendType_;
+  std::string pg_desc_;
 
   // Debug level setting. It is parsed once when ProcessGroup is constructed and
   // remains the same across use of this process group.