enabling diverse datasets in val / test
# What does this PR do ?

Allows to specify multiple datasets with different configs  in validation and test 

ALL

# Changelog 
When processing val / test dataset configs, if a dict is detected instead of a list, overload the entire dataset config, not just the manifest path.

# Usage
Specify multiple, different dataset configs as shown below.

```yaml
  validation_ds:
    ds_item:
    - name: en
      manifest_filepath:
      - ${d}/en/val_test/fisher/audio_manifest_dev_clean_en.json
      - ${d}/en/val_test/europarl/audio_manifest_dev_clean_en.json
      sample_rate: ${model.sample_rate}
      batch_size: 16
      shuffle: false
      num_workers: 8
      pin_memory: true
      max_duration: 20.0
      min_duration: 0.1
      use_start_end_token: false
    - name: es
      manifest_filepath:
      - ${d}/es/nemo_sp_asr_set_3pt0/dev/fisher/dev_fisher_manifest_es.json
      - ${d}/es/nemo_sp_asr_set_3pt0/dev/mcv12/dev_mcv12_manifest_es.json
      - ${d}/es/nemo_sp_asr_set_3pt0/dev/mls/dev_mls_manifest_es.json
      - ${d}/es/nemo_sp_asr_set_3pt0/dev/voxpopuli/dev_voxpopuli_manifest_es.json
      sample_rate: ${model.sample_rate}
      batch_size: 32
      shuffle: false
      num_workers: 8
      pin_memory: true
      max_duration: 10.0
      min_duration: 1.0
      use_start_end_token: false

```

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

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[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
