Broaden support of unflatten to split graphs

### 🚀 The feature, motivation and pitch

I hit the following `AssertionError` when trying to unflatten a submodule created by fx `split_module`.
```
[rank0]: File "torch/distributed/pipelining/_IR.py", line 750, in _from_traced
[rank0]: new_submod = _outline_submodules(submodule.graph)
[rank0]: File "torch/distributed/pipelining/_unflatten.py", line 27, in _outline_submodules
[rank0]: ).run_outer()
[rank0]: File "torch/export/unflatten.py", line 1373, in run_outer
[rank0]: self.run_from(node_idx)
[rank0]: File "torch/export/unflatten.py", line 1389, in run_from
[rank0]: assert node.op != "placeholder", f"""
[rank0]: AssertionError:
[rank0]: node idx 1: gate_weight
[rank0]: node idx 2: submod_1 <---
[rank0]: node idx 3: lifted_tensor_0
[rank0]: node idx 4: lifted_tensor_1
```

### Root Cause
(i) It seems that the unflatten function assumes that placeholders are the first-n nodes of the graph. It thus pre-processes the placeholder nodes until a first non-placeholder one, and starts `run_from` from there. In the `run_from` function, it asserts that no node is a `placeholder`.

(ii) However, when `split_module` creates submodules, it may not necessarily put all placeholder nodes as first-n nodes. In this above case, `submod_1` is a `get_attr` node **surrounded by** in a bunch of `placeholders`.

(iii) Here, `submod_1` is a HOP created by torch.export, before graph split.

### Alternatives

If there is a way to lint the split graph so that all the first-n nodes are placeholders, please let me know.

### Additional context

Am calling unflatten on subgraph for use by pipeline parallel. The above only occurs when there is a HOP in the subgraph.

cc @chauhang @penguinwu @avikchaudhuri @gmagogsfm @zhxchen17 @tugsbayasgalan @angelayi @suo @ydwu4 @H-Huang @awgu @wanchaol @fegin @fduwjj @wz337 @wconstab @d4l3k @pragupta @msaroufim @dcci