fx quant: split linear test cases (#49740)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49740
1. Separates the module and functional linear test cases.
2. Combines the test case which tests for linear bias observation into
the main linear test case, as requested in
https://github.com/pytorch/pytorch/pull/49628.
Test Plan:
```
python test/test_quantization.py TestQuantizeFxOps.test_linear_module
python test/test_quantization.py TestQuantizeFxOps.test_linear_functional
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D25681272
fbshipit-source-id: 0ed0ebd5afb8cdb938b530f7dbfbd79798eb9318
