ns for fx: add partial support for subgraphs with base_op_node (#54254)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54254
In fp16 emulation, we now have patterns such as
```
... -> dequantize -> linear -> relu -> to(torch.float16) -> ...
```
This PR adds support for
* specifying a subgraph's "base_op_node", which is the node with the op
which should be matched to related nodes. In the example above,
"base_op_node" would be the linear node, and it would be the second
node in the matched pattern.
* matching these fusion patterns and properly setting "base_op_node"
based on pattern and index
* using "base_op_node" instead of "start_node" throughout the NS
codebase wherever the intent is to match subgraphs or create names
for subgraphs.
At the end of this PR, matching unshadowed activations with an example
fp16 emulation pattern works e2e.
I'm saving the following work for future PRs (soon), mostly to keep
PR size manageable:
* adding weight matching (will require some changes to function which
extracts weights)
* adding shadowed activation matching (will require some changes to
shadow copying)
* adding input logging for these patterns (will likely require some changes as well)
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs.test_linear_fp16
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D27158199
fbshipit-source-id: 49fc445395452fda62e3c7a243544190f9af691c
