[audio] Keep input directory structure when saving processed files
<!-- pylint-output-strict-mode-false -->

beep boop 🤖: 🙏 The following files have warnings. In case you are familiar with these, please try helping us to improve the code base.

---

Your code was analyzed with PyLint. The following annotations have been identified:

```
************* Module process_audio
examples/audio/process_audio.py:42:0: C0301: Line too long (122/119) (line-too-long)
examples/audio/process_audio.py:78:0: C0115: Missing class docstring (missing-class-docstring)
examples/audio/process_audio.py:115:0: C0116: Missing function or method docstring (missing-function-docstring)
************* Module nemo.collections.audio.models.audio_to_audio
nemo/collections/audio/models/audio_to_audio.py:346:0: C0301: Line too long (137/119) (line-too-long)
nemo/collections/audio/models/audio_to_audio.py:347:0: C0301: Line too long (136/119) (line-too-long)
nemo/collections/audio/models/audio_to_audio.py:348:0: C0301: Line too long (150/119) (line-too-long)
nemo/collections/audio/models/audio_to_audio.py:41:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/audio/models/audio_to_audio.py:126:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:129:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:133:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:137:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:145:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:153:4: C0116: Missing function or method docstring (missing-function-docstring)

-----------------------------------
Your code has been rated at 9.63/10
```

---

Thank you for improving NeMo's documentation! <!-- pylint-output-strict-mode-false -->

beep boop 🤖: 🙏 The following files have warnings. In case you are familiar with these, please try helping us to improve the code base.

---

Your code was analyzed with PyLint. The following annotations have been identified:

```
************* Module process_audio
examples/audio/process_audio.py:42:0: C0301: Line too long (122/119) (line-too-long)
examples/audio/process_audio.py:78:0: C0115: Missing class docstring (missing-class-docstring)
examples/audio/process_audio.py:115:0: C0116: Missing function or method docstring (missing-function-docstring)
************* Module nemo.collections.audio.models.audio_to_audio
nemo/collections/audio/models/audio_to_audio.py:346:0: C0301: Line too long (137/119) (line-too-long)
nemo/collections/audio/models/audio_to_audio.py:347:0: C0301: Line too long (136/119) (line-too-long)
nemo/collections/audio/models/audio_to_audio.py:348:0: C0301: Line too long (150/119) (line-too-long)
nemo/collections/audio/models/audio_to_audio.py:41:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/audio/models/audio_to_audio.py:126:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:129:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:133:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:137:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:145:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/audio/models/audio_to_audio.py:153:4: C0116: Missing function or method docstring (missing-function-docstring)

-----------------------------------
Your code has been rated at 9.63/10
```

---

Thank you for improving NeMo's documentation!
# What does this PR do ?

This PR modifies the processing script to preserve the directory structure when saving output files.

**Collection**: audio

# Changelog 
- Updated processing script to resolve `input_dir` based on the input manifest and individual filepaths
- Added an `input_dir` option to `process`

# Usage
N/A

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