wav2vec ASR
**Describe the bug**

I have been trying to get wav2vec ASR training up and running. I am mainly trying to use the model defined in

```
examples/asr/experimental/wav2vec/configs/wav2vecCTC.yaml
```

I also tried to get a model with wav2vec_pretrain.yaml running, but I get the same error as outlined below.

The call I use is:

```
python /workspace/nemo/examples/asr/asr_ctc/speech_to_text_ctc.py \
  --config-path=/workspace/nemo/examples/asr/experimental/wav2vec/configs \
  --config-name=wav2vecCTC \
  exp_manager.name=wav2vec \
  +exp_manager.exp_dir=/experiments \
  +exp_manager.version=20220613-1105 \
  exp_manager.resume_if_exists=true \
  exp_manager.resume_ignore_no_checkpoint=true \
  +exp_manager.checkpoint_callback_params.save_best_model=true \
  model.train_ds.manifest_filepath=/data/train.nemo \
  model.train_ds.batch_size=16 \
  model.train_ds.max_duration=17 \
  +model.train_ds.min_duration=1 \
  model.validation_ds.manifest_filepath=/data/validation.nemo \
  model.validation_ds.batch_size=16 \
  model.test_ds.manifest_filepath=/data/test_old.nemo \
  model.test_ds.batch_size=16 \
  model.tokenizer.dir=/data/out/tokenizer_spe_bpe_v128 \
  model.tokenizer.type=bpe
```

The error is get is:

```
KeyValidationError('ListConfig indices must be integers or slices, not str\n    full_key: [0][emb_dim]\n    object_type=list')
```

It seems like something is wrong with `emd_dim`. I leave this at its default in the `embedding_dim` parameter in the YAML file. Even if I set this explicitly I get the same error. Attached is also the full output of the call: [output_20220613-1105.txt](https://github.com/NVIDIA/NeMo/files/8890509/output_20220613-1105.txt).

Another minor thing I also noticed is that the command call crashes even earlier if I do not provide a test set. Based on the YAML file test set should not be mandatory as it is set to null and not ??.

I am aware that this is an experimental functionality, so any help will be greatly appreciated.

**Steps/Code to reproduce bug**

Full command call can be found in the previous section.

**Expected behavior**

I expect the wav2vec model to start the training process.

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

I am using the official NVIDIA pytorch 22.01 container, the used NeMo version is the current version of the master branch.

**Environment details**

I am using the official NVIDIA pytorch 22.01 container, the used NeMo version is the current version of the master branch.

**Additional context**

Will provided any additional info if required.
