import nemo.collections.nlp fails
**Describe the bug**
Hi, exec `python examples/nlp/duplex_text_normalization/google_data_preprocessing.py --data_dir=/ssd5/exec/shenchengen/data/tn/ --output_dir=preprocessed/ --lang=en`, there has import error：
```
Traceback (most recent call last):
  File "google_data_preprocessing.py", line 42, in <module>
    from helpers import flatten
  File "/ssd5/exec/shenchengen/software/NeMo/examples/nlp/duplex_text_normalization/helpers.py", line 18, in <module>
    from nemo.collections.nlp.models import DuplexDecoderModel, DuplexTaggerModel
  File "/ssd5/exec/shenchengen/software/NeMo/nemo/collections/nlp/__init__.py", line 15, in <module>
    from nemo.collections.nlp import data, losses, models, modules
  File "/ssd5/exec/shenchengen/software/NeMo/nemo/collections/nlp/models/__init__.py", line 16, in <module>
    from nemo.collections.nlp.models.duplex_text_normalization import (
  File "/ssd5/exec/shenchengen/software/NeMo/nemo/collections/nlp/models/duplex_text_normalization/__init__.py", line 15, in <module>
    from nemo.collections.nlp.models.duplex_text_normalization.duplex_decoder import DuplexDecoderModel
  File "/ssd5/exec/shenchengen/software/NeMo/nemo/collections/nlp/models/duplex_text_normalization/duplex_decoder.py", line 25, in <module>
    import nemo.collections.nlp.data.text_normalization.constants as constants
  File "/ssd5/exec/shenchengen/software/NeMo/nemo/collections/nlp/data/text_normalization/__init__.py", line 14, in <module>
    from nemo.collections.nlp.data.text_normalization.decoder_dataset import TextNormalizationDecoderDataset
  File "/ssd5/exec/shenchengen/software/NeMo/nemo/collections/nlp/data/text_normalization/decoder_dataset.py", line 21, in <module>
    import nemo.collections.nlp.data.text_normalization.constants as constants
AttributeError: module 'nemo.collections' has no attribute 'nlp'
```

then I use ipython, also hase AttributeError: module 'nemo.collections' has no attribute 'nlp'

Installed by cloning the git repo and running ./reinstall.sh. I have also tried installing via` pip install nemo_toolkit['all']`, and `pip install git+https://github.com/NVIDIA/NeMo.git@$BRANCH#egg=nemo_toolkit[all]`.  .These all produced the same result.

environment:
python 3.6.13 nemo 1.2.0 