Add observer nodes for input data nodes (#19232)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19232
Add observer nodes to collect stats for input data nodes excluding params
which are constant at inference and need not be observed. This information
is required to compute quantization params.
Differential Revision: D14885485
fbshipit-source-id: 8762cc2a4e510e1553b3dbd1d1aecd55b4bdb89f
