Extract independent snapshot functions for metrics: network, RL, experiment, statistics etc.

[zuoxingdong]

This will dramatically increase reusability and reduce boilerplate code.

e.g.

snapshot_nn(net, ['total_parameters', 'trainable_parameters'])

Networks:

• Total parameters: trainable parameters, un-trainable parameters
  • Extract the number of such parameters
  • Extract a generator of such parameters
• Gradient norm

Policy:

• Entropy