Parses the encoded key to keys and weights.

This parse function will remove all spaces. Different keys are split by "," and then weight associated with key is split by ":".

key A string represents a key, or a string of multiple keys, split by ",", and weighted by the weights split by ":". For example, key = 'softmax_loss:0.9,sigmoid_cross_entropy_loss:0.1'.

A dict from keys to weights.