Add N-distill

[pwuethri]

Adding N-distill according to https://arxiv.org/abs/1902.02186

• Add next observation to trajectory data structure
• Directly compute gradient using the given update rule (This is the difference compared to Teacher distill, on-policy distill and entropy regularised distillation
• Update nn parameters accordingly
• Test is policy distillation works using available teacher policy

[pwuethri]

Script exits with
terminate called after throwing an instance of 'std::runtime_error what(): invalid argument 13: ldc should be at least max(1, m=0), but have 0 at /pytorch/aten/src/TH/generic/THBlas.cpp:334 [1] 24572 abort (core dumped) python run_n_distill.py
-> need to check the computed gradients