[layer] Add first draft of lstm cell

[layer] Add first draft of lstm cell

This patch adds the first draft of lstm cell. This has certain
advantages over the existing LSTM when executing for single timestep:
- does not need to divide the data over smaller chunks for computation.
This saves the overhead of creating tensors, an extra loop and combines
computation to a single call for efficiency.
- some of the operations have been avoided based on the current timestep
(which were zeros).
More optimizatios will be coming soon for LSTM cell.

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>