requirements_nlp.txt is not enough
**Describe the bug**

Installing `requirements_nlp.txt` is not enough to import `nemo.collections.nlp`.

**Steps/Code to reproduce bug**

1. Create new `conda` `Python 3.8` environment
2. Install PyTorch 1.8.2
3. Run `python -m pip install git+https://github.com/NVIDIA/NeMo.git@main#egg=nemo_toolkit[nlp]`
4. Run code
```python
from nemo.collections import nlp as nemo_nlp
```
5. You will get traceback
```
[NeMo W 2021-08-17 21:51:45 optimizers:47] Apex was not found. Using the lamb optimizer will error out.
[NeMo W 2021-08-17 21:51:46 __init__:22] `pynini` is not installed !
    Please run the `nemo_text_processing/setup.sh` scriptprior to usage of this toolkit.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo/collections/nlp/__init__.py", line 15, in <module>
    from nemo.collections.nlp import data, losses, models, modules
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo/collections/nlp/models/__init__.py", line 16, in <module>
    from nemo.collections.nlp.models.duplex_text_normalization import (
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/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 "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo/collections/nlp/models/duplex_text_normalization/duplex_decoder.py", line 21, in <module>
    from nemo_text_processing.text_normalization.normalize_with_audio import PYNINI_AVAILABLE, NormalizerWithAudio
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo_text_processing/text_normalization/normalize_with_audio.py", line 25, in <module>
    from nemo.collections.asr.metrics.wer import word_error_rate
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo/collections/asr/__init__.py", line 15, in <module>
    from nemo.collections.asr import data, losses, models, modules
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo/collections/asr/modules/__init__.py", line 15, in <module>
    from nemo.collections.asr.modules.audio_preprocessing import (
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo/collections/asr/modules/audio_preprocessing.py", line 24, in <module>
    from nemo.collections.asr.parts.preprocessing.features import FilterbankFeatures
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo/collections/asr/parts/preprocessing/__init__.py", line 16, in <module>
    from nemo.collections.asr.parts.preprocessing.features import (
  File "/home/lab/anaconda3/envs/tmp/lib/python3.8/site-packages/nemo/collections/asr/parts/preprocessing/features.py", line 37, in <module>
    import librosa
ModuleNotFoundError: No module named 'librosa'
```

**Expected behavior**

No errors

**Environment overview (please complete the following information)**

 - Environment location: Bare metal
 - Method of NeMo install: `python -m pip install git+https://github.com/NVIDIA/NeMo.git@main#egg=nemo_toolkit[nlp]`

**Environment details**

If NVIDIA docker image is used you don't need to specify these.
Otherwise, please provide:
- OS version: Ubuntu 20.04.2 LTS
- PyTorch version: 1.8.2
- Python version: 3.8
