Add path_prefix to concat to the begining of audio_file
# What does this PR do ?

Add path_prefix to concat to the begining of audio_file.

**Collection**: ASR collection

# Changelog 
- Add path_prefix to concat to the begining of audio_file.

# Usage
* You can use path_prefix on the cloud platforms like AWS Sagemaker, where you don't know the full path of the data before the training process is started. 
* Ones training is started you get path of data from environment variables and provide to path_prefix script parameter

```python
# python3 examples/asr/asr_ctc/speech_to_text_ctc.py +model.train_ds.path_prefix=your/data/path +model.validation_ds.path_prefix=your/data/path
```

**PR Type**:
-  New Feature
