fx quant: fix edge case with copynode after user function (#55710)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55710
In the current code, there is an edge case which leads to an error
after the prepare step:
1. have a pattern like this:
```
user_func_unmatched_to_qhandler -> node_matched_to_copy_node_qhandler
```
2. the user function returns a type which is not observable (i.e. not a
Tensor)
3. if this is run through `prepare_fx`, calibrating it with data leads
to a runtime error, because observers cannot observe non-tensor types.
This PR fixes the issue. If a node matched to `CopyNodeQuantizeHandler`
is after an unmatched node, we delete the observer.
Test Plan:
```
python test/test_quantization.py TestQuantizeFx.test_no_obs_between_unmatched_node_and_copy_node
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D27686811
fbshipit-source-id: 320be41b1f383c6352ff89fb39a9f480822a3bb2
