Add size information

[rmchurch]

Added information for estimating the total size of the model. Estimates taken from here: http://jacobkimmel.github.io/pytorch_estimating_model_size/ and associated github repo
-calculates size of input, parameters, and forward/backward pass intermediate variables
-prints out these estimates and total
-batch_size optional input

Sample output:

================================================================
Total params: 132,710
Trainable params: 132,710
Non-trainable params: 0

---

Input size (MB): 0.10
Forward/backward pass size (MB): 735.99
Params size (MB): 0.51
Estimated Total Size (MB): 736.59

---

[sksq96]

This post was a great read. Thanks for adding this to the package!

[jamespy007]

Thanks for the tool you provided! Here is my question: will pytorch-summary working for GRU or other RNN module?