[ao] fixing multihead attention convert size (#110407)
Summary: after converting nn.multihead attention we weren't deleting the
old in_proj_weight and in_proj_bias despite not (really) using them.
Test Plan: python test/test_quantization.py -k
"test_custom_module_multi_head_attention"
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Pull Request resolved: https://github.com/pytorch/pytorch/pull/110407
Approved by: https://github.com/jerryzh168
