op to gen quant params from min-max thresholds
Summary: Adding support to gen qparams to quantize a tensor from min and max thresholds of a tensor
Test Plan:
```
buck test mode/opt caffe2/caffe2/quantization/server:int8_gen_quant_params_min_max_test
```
```
Started reporting to test run: https://our.intern.facebook.com/intern/testinfra/testrun/5629499573509506
✓ ListingSuccess: caffe2/caffe2/quantization/server:int8_gen_quant_params_min_max_test - main (2.522)
✓ Pass: caffe2/caffe2/quantization/server:int8_gen_quant_params_min_max_test - test_int8_gen_quant_params_min_max_op (caffe2.caffe2.quantization.server.int8_gen_quant_params_min_max_test.TestInt8GenQuantParamsMinMaxOperator) (1.977)
Summary
Pass: 1
ListingSuccess: 1
```
Reviewed By: hx89
Differential Revision: D24485985
fbshipit-source-id: 18dee193f7895295d85d31dc013570e5d5d97357
