Fixed Apex guard when imported classes are used for default values
  1. Added ApexGuardDefaults class that provides None attributes when an Apex class is missing.
  2. Fixed Apex guard.

Signed-off-by: Micha Livne <mlivne@nvidia.com>

# What does this PR do ?

This PR fixes an Apex guard that leaves constructors with undefined constructor values.

**Collection**: [Note which collection this PR will affect]

# Changelog 
- Added `ApexGuardDefaults` class to `nemo/collections/nlp/modules/common/megatron/utils.py`
- Used `ApexGuardDefaults` to replace missing classes in `nemo/collections/nlp/modules/common/megatron/transformer.py`