Add some improvements in Chapter 7

[mtndaghyani]

These changes have been applied to Chapter 7 materials:

1. In RNN.pdf slides, it was not clear that the calculations on page 48/69 was related to the Vanilla RNN network , not LSTM network (which is on page 49/69). To clarify it, a statement is added to this slide.
2. To better demonstrate the formulations that are used on page 49/69 for LSTM, a related image is added. (Similar to the image on page 47/69 for RNN).
3. The GRU architecture image is added to the LSTM_GRU_Bidirectional.ipynb, similar to the existing LSTM image.

[mahsayazdani]

@mtndaghyani Your pull request has been merged. Please email me your student ID.