Implement LpNorm regularizer to be used on the inputs for feature importance (#26376)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26376
* Create the new dense_feature_reg (FCInputLpNorm) for feature importance to be applied to the fully-connected layer for feature-importance.
Test Plan: * Unit test located in: `caffe2/caffe2/fb/dper/layer_models/tests/split_1/sparse_nn_test.py`
Reviewed By: un-disclosed
Differential Revision: D17360361
fbshipit-source-id: 1a0e119eeb17199a13dfffe58b3036ea4255e301
