[quant][improvement] Added quantized fill test for per channel quantized tensors
Summary:
Previously, the quantization test suite only tested the fill operator
for per tensor quantized tensors. This PR adds a test case for per
channel quantized tensors.
The existing `test_qtensor_fill`, which the newly introduced
test function `test_qtensor_fill_per_channel` is based on, case had some ambiguous
naming conventions. This PR renames some of those variables to be
clearer.
Test Plan:
```
python test/test_quantization.py test_qtensor_fill_per_channel
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78661
Approved by: https://github.com/jerryzh168
