Add pinned pynini and scipy installs to TTS training tutorial
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Fixes some execution errors for the training notebook (in Colab) by adding & pinning installs.

**Collection**: TTS

# Changelog 
- Pynini pinned to version 2.1.4 (needed for training)
- Scipy pinned to version 1.7.3 (fixes comparison issue from #3961)
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

# Additional Information
* Related to #3961 
