Adjust stochastic depth dropout probability calculation
# What does this PR do?

This PR adjusts calculation of stochastic depth dropout probability.
Currently, calculation fails for 1-layer ConformerEncoder with the default setup.

The culprit is calculation of the probability in `compute_stochastic_depth_drop_probs ` for `linear` mode:

```
layer_drop_probs += [l / (L - 1) * stochastic_depth_drop_prob for l in range(L)]
```

since in this case `L-1 == 0`.

## Proposed solution

Change calculation for the linear mode to the following
```
# we start with 1/L * drop_prob and and end with the desired drop probability.
layer_drop_probs += [l / L * stochastic_depth_drop_prob for l in range(1, L + 1)]
```

This can handle setup with a single layer and is consistent with the probability calculation for the `uniform` mode.

### Example
An example of calculated probabilities:

```
num_layers = 7
stochastic_depth_start_layer = 3
stochastic_depth_drop_prob = 0.5
```

| layer |  0  |  1  |  2  |  3  |  4  |  5  |  6  |
| ----- | --- | --- | --- | --- | --- | --- | --- |
| uniform         | 0.0 | 0.0 | 0.0 | 0.5   | 0.5   | 0.5   | 0.5 |
| linear original | 0.0 | 0.0 | 0.0 | 0.0   | 0.167 | 0.333 | 0.5 |
| linear proposed | 0.0 | 0.0 | 0.0 | 0.125 | 0.25  | 0.375 | 0.5 |



**Collection**: ASR

# Changelog 
- Add specific line by line info of high level changes in this PR.

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# Additional Information
* Related to # (issue)
