Update ASR torch.stft() call to use return_complex=True
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Updates the remaining call to `torch.stft()` to use `return_complex=True` due to the deprecation of the `return_complex=False` option.

I've done a quick grep through the NeMo repo, this should be the only other place where `torch.stft()` (or `torch.istft()`) is called.

**Collection**: ASR

# Changelog 
- Changed the preprocessing lambda to use `return_complex=True`, which means `torch.stft()` will always return a complex tensor
- Removed the conditional for `view_as_real` since it will always need to be called now

**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

# Additional Information
* See #5729 for more information (and the TTS fixes)
