[quant] Implemented InputWeightObserver for Linear inputs
Summary: Implemented two observers (InputEqualObserver and WeightEqualObserver) which will be inserted into the graph during prepare_fx().
Test Plan: python test/test_quantization.py TestEqualizeFx
Reviewed By: supriyar
Differential Revision: D28836954
fbshipit-source-id: 25517dc82ae67698ed8b2dc334e3323286976104
