Spec augmentation time masking
**Describe the bug**

I was wondering if this line of [code](https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/asr/parts/numba/spec_augment/spec_aug_numba.py#L286) for time masking of spec augmentation is correct.
Unless intended, it looks more reasonable with `time_starts = torch.cat(time_starts, 0)` rather than `time_starts = torch.cat(time_lengths, 0)` as this line of code makes `time_starts` and `time_lengths` equivalent to each other.

**Steps/Code to reproduce bug**

Any scripts useing spec augmentation, e.g., `python speech_to_text_bpe.py --config-path conf/conformer --config-name conformer_ctc_bpe`

**Expected behavior**

It currently generates time masks only at the beginning of the sequence.
![Screenshot from 2021-12-07 22-47-11](https://user-images.githubusercontent.com/50283958/145161693-8069705e-3599-494c-89b4-70eec6140545.png)

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

`pip install -e .`

**Environment details**

N/A

**Additional context**

N/A
