Fix incorrect masking that always included all samples.
The current nemo version of the tdnn masking performs comparison as follows:
```
    lens_mat = torch.arange(max_len).to(device)
    lens = lens * max_len
    mask = lens_mat[:max_len].unsqueeze(0) < lens.unsqueeze(1)
```

If we expand the comparison and simplify device and unsqueeze calls , we get

```
torch.arange(max_len)[:max_len] < lens * max_len
```

This will be always True as long as each element of `lens` is larger than 1.  The multiplication by `max_len` is not needed. 

The bug can also be confirmed by running the test present in the PR after undoing the fix. 

Signed-off-by: Aleksei Kalinov <alekseia@nvidia.com>