[quant][pt2] Handle maxpool + conv + bn case in prepare QAT (#100941)
Summary: This commit fixes a bug where we copy the metadata from
the wrong node after replace_pattern. This happened in the case
of [maxpool -> getitem1 -> conv -> bn -> getitem2], where
`getitem1` is the placeholder node fed into the fused conv + bn
pattern, and we incorrectly copied the metadata from `getitem1`
instead of from `getitem2`. We fix this bug by filtering out
the placeholder nodes before doing the metadata copying.
Test Plan:
python test/test_quantization.py TestQuantizePT2E.test_prepare_qat_conv_bn_fusion_getitem_placeholder
Reviewers: jerryzh168, kimishpatel
Differential Revision: [D45916751](https://our.internmc.facebook.com/intern/diff/D45916751)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100941
Approved by: https://github.com/jerryzh168
