Use noop observer to pass dtype for dynamic quantization (#26709)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26709
Polishes implementation from #25975. Primarily, we use NoopObserver to communicate that weights need to be quantized to float16. The very top-level API (quantize_dynamic) stays the same with `dtype` argument but the implementation follows the common flow.
One can argue that dynamic fp16 quantization doesn't really fit into the 'observer' mechanism. It's in fact not ideal, but it's better to have the same flow than branching on both dtype and qconfig.
Test Plan: Imported from OSS
Differential Revision: D17544103
Pulled By: dzhulgakov
fbshipit-source-id: 6af3f18c35929a1a53ea734079c005f656e4925f
