[quant] Add FP16Observer for fp16 quant support (#42221)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42221
Adds a new observer that emits a warning if the range of tensor is beyond fp16 range. This will be further used in graph mode quantization to insert the cast to fp16 ops in the graph
Test Plan:
python test/test_quantizaton.py TestObserver.test_fp16_observer
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D22849222
fbshipit-source-id: a301281ce38ba4d4e7a009308400d34a08c113d2
