default value of use_local_stats in batch norm layer

[wyli]

It seems that in many window-based segmentation/regression use cases, either settingis_training=True or setting use_local_stats=True in the batch normalisation layer at inference time leads to better performance than those with NiftyNet's default batch norm options.

Therefore this ticket proposes to add a use_local_stats option in the config file, and make use_local_stats=True as the default for segmentation application with batch norm.

Specifically, in the current implementation (v0.4.0) the default values are:

• training: is_trainign=True use_local_stats=True
• inference: is_training=False use_local_stats=True
  which uses running averages from training at inference time, following Ioffe and Szegedy (2015) https://arxiv.org/abs/1502.03167.

The proposed change will use these values by default:

• training: is_trainign=True use_local_stats=True
• inference: is_training=False use_local_stats=True
  which will effectively follow Ulyanov et al. (2017) https://arxiv.org/abs/1607.08022.

This ticket is related to #280, #285, #282.

thoughts?

Probably to be addressed with #282