Refactor get_node_args and friends into a class (pytorch#2741)

Summary:

Torchrec rewriting logic got a bit hairy over the years, this sequence of changes aims to refactor the rewrite logic to be less convoluted and more maintainable in the future.

This change: _get_node_args and related functions pass around lot of "context" (train_pipeline_context, streams, etc.) that rarely or never changes + some "state" (model, pipelined_preprocs) that is accumulated during the run. Refactoring `_get_node_args` (and friends) into a class allows initializing/passing those into class constructor, and simplifies the call signatures a lot

Internal

Diff stack navigation:
1. D69292525 and below - before refactoring
2. D69438143 - Refactor get_node_args and friends into a class (**you are here**)
3. D69461227 - refactor "joint lists" in ArgInfo into a list of ArgInfoStep
4. D69461226 - refactor `_build_args_kwargs` into instance methods on ArgInfo and ArgInfoStep
5. D69461228 - split monolithic `ArgInfoStep` into a class hierarchy

Differential Revision: D69438143

torchrec/distributed/train_pipeline/tests/test_train_pipelines_utils.py

@@ -24,9 +24,9 @@
 )
 from torchrec.distributed.train_pipeline.utils import (
     _build_args_kwargs,
-    _get_node_args,
     _rewrite_model,
     ArgInfo,
+    NodeArgsHelper,
     PipelinedForward,
     PipelinedPostproc,
     TrainPipelineContext,
@@ -367,10 +367,9 @@ def test_get_node_args_helper_call_module_kjt(self) -> None:
             {},
         )
 
-        num_found = 0
-        _, num_found = _get_node_args(
-            MagicMock(), kjt_node, set(), TrainPipelineContext(), False
-        )
+        node_args_helper = NodeArgsHelper(MagicMock(), TrainPipelineContext(), False)
+
+        _, num_found = node_args_helper.get_node_args(kjt_node)
 
         # Weights is call_module node, so we should only find 2 args unmodified
         self.assertEqual(num_found, len(kjt_args) - 1)