Fix for quantization modifier w/ DDP

[anmarques]

Removes "module." from submodule names inserted by DDP.

Test plan:
Re-quantized the pruned 80% RN50 model with the following quantization modifier:

  - !QuantizationModifier
    start_epoch: eval(quant_start_epoch)
    disable_quantization_observer_epoch: eval(quant_disable_observer_epoch)
    freeze_bn_stats_epoch: eval(quant_freeze_bn_epoch)
    ignore: ['Softmax', 'classifier']

Before this PR the quantization modifier failed to match the "classifier" module name because under DDP this module was renamed as module.classifier