[IG] Avoid generation of empty merge cpu submodule by splitter v2 (pytorch#140794)

Summary:
Customize splitter behavior to mark `get_attr` nodes as acc supported.
Currently these nodes are excluded by `FxNetAccNodesFinder` which marks all nodes with op not in `CALLABLE_NODE_OPS` ("call_module", "call_function", "call_method") as unsupported.

Before this change, merge-net is split into an almost empty cpu submodule with a single empty output node:
```
INFO:caffe2.torch.fb.model_transform.experimental.prepare_fx_model:###### debug_print nodes for _run_on_cpu_0
INFO:caffe2.torch.fb.model_transform.experimental.prepare_fx_model:Found output node: n.name='output', n.target='output', n.args=((),), n.kwargs={}, n.meta={}
INFO:caffe2.torch.fb.model_transform.experimental.prepare_fx_model:return ()
INFO:caffe2.torch.fb.model_transform.experimental.prepare_fx_model:
_run_on_cpu_0 stats for merge:
[output] output: 1
```
full log: P1678727348 (generated using same command as below)

Test Plan:
Tested by lowering `ig_organic_feed_cn_v2_mtml` using cmd:
```
buck run mode/opt-split-dwarf //tgif/cli:cli -- --model-name=ig_organic_feed_cn_v2_mtml --model-type ig_organic_feed_cn_v2_mtml --world-size=1 --storage-mode 1 --inference-dtype=FP16 --meta-transform=False --use-random-weights=True --accelerator-arch=3 --enable-input-dist=True --embedding-tables-dtype=FP16 --mtia-use-torch-export=True embedding-quantization-pass torchrec-sharding-pass tgif-split-pass gen-app-graph-pass tgif-mtia-lowering-pass dense-quantization-pass save-torch-package-pass generate-model-package-pass pack-weights-and-save-pass 2>&1 | tee /tmp/publish_ig_organic_feed_cn_v2_mtml_mtia_export_20241114_splitter_2.log
```
Output shows only 1 acc submodule is generated for merge:
```
INFO 18:33:15.951 1735650 utils.py:235: [TGIF] num of acc submodules: 1
INFO 18:33:15.952 1735650 utils.py:236: [TGIF] num of cpu submodules: 0
INFO 18:33:16.534 1735650 logging_utils.py:53: [TGIF] _run_on_acc_0 graph module debug info: https://www.internalfb.com/intern/everpaste/?color=0&handle=GK4VKhWsDKF9VdsDAKxhR6KAlhJ0br0LAAAz
INFO 18:33:16.534 1735650 utils.py:257: [TGIF] Start MTIA lowering _run_on_acc_0 in merge, device ordinal: -1
```
full log: P1679596796

Differential Revision: D65983916

Pull Request resolved: pytorch#140794
Approved by: https://github.com/ezyang

Author: sidt-meta

torch/fx/passes/splitter_base.py

@@ -331,6 +331,7 @@ def __init__(
         settings: _SplitterSettingBase,
         non_acc_submodule_name: str = "_run_on_cpu_",
         return_tuple: bool = False,
+        nodes_finder: Optional[FxNetAccNodesFinder] = None,
     ):
         """
         Preprocesses graph before splitting:
@@ -348,9 +349,11 @@ def __init__(
         self.settings = settings
         self.operator_support = operator_support
         self.sample_input = sample_input
-        self.acc_nodes = FxNetAccNodesFinder(
-            self.module, self.operator_support, self.settings.allow_non_tensor
-        )()
+        if nodes_finder is None:
+            nodes_finder = FxNetAccNodesFinder(
+                self.module, self.operator_support, self.settings.allow_non_tensor
+            )
+        self.acc_nodes = nodes_finder()
 
         if self.settings.skip_fusion:
             self.fusions = {}