Experiment manager step timing
This PR adds the ability to log the timing of a step for train/validation/test modes during training.
This feature is added to all NeMo models.
Time is measured in seconds.

# Configuration

The following config allow to control logging of step timing:

```
exp_manager:
  log_step_timing: True # logging is enabled by default
  step_timing_kwargs:
    reduction: "mean" # reduction for multiple measurements
    sync_cuda: False # if True calls torch.cuda.synchronize() for start/stop of measurement. Disabled by default
```

By default the logging is turned on. In order to turn off the feature, the following config can be used:

```
exp_manager.log_step_timing=False
```

By default cuda sync is turned off. In order to turn it on, the following config can be used:
```
exp_manager.step_timing_kwargs.sync_cuda=True
```

# Logging

The following keys will be logged in the corresponding mode (when available): `train_step_timing`,   `train_backward_timing`, `validation_step_timing`,   `test_step_timing`.

## Weights & Biases Output Example

![image](https://user-images.githubusercontent.com/7453913/135684915-16900e7c-b079-4db9-898d-f73ddce12754.png)

## Pytorch Lightning Training Loop

Below is `pytorch lightning` training loop, in order to better understand what exactly is measured:

```
def train_loop():
    on_epoch_start()
    on_train_epoch_start()

    for batch in train_dataloader():
        on_train_batch_start()

        on_before_batch_transfer()
        transfer_batch_to_device()
        on_after_batch_transfer()

        training_step()

        on_before_zero_grad()
        optimizer_zero_grad()

        on_before_backward()
        backward()
        on_after_backward()

        on_before_optimizer_step()
        optimizer_step()

        on_train_batch_end()

        if should_check_val:
            val_loop()
    # end training epoch
    training_epoch_end()

    on_train_epoch_end()
    on_epoch_end()
```

See <https://pytorch-lightning.readthedocs.io/en/latest/common/lightning_module.html#hooks> for full details.