Clarify comments and neural types in Model Primer Notebook 
# Changelog
- Address Neural Type comments from https://github.com/NVIDIA/NeMo/issues/1634 regarding LSTM
- Address clarification of "GPT Encoder neural module" vs "Transformer Decoder network" issue raised in https://github.com/NVIDIA/NeMo/issues/1638

Closes https://github.com/NVIDIA/NeMo/issues/1634 and https://github.com/NVIDIA/NeMo/issues/1638

Signed-off-by: smajumdar <titu1994@gmail.com>