Fix for crashes with tensorboard_logger=false and VP + LoRA
# What does this PR do ?

Two tiny fixes for crashes that can occur with with (1) VP + LoRA and (2) exp_manager.create_tensorboard_logger=false.

**Collection**: nlp

# Changelog 
- With virtual pipeline parallel + lora it is currently possible to encounter:
```
  File "/opt/NeMo/nemo/collections/nlp/parts/mixins/nlp_adapter_mixins.py", line 477, in on_load_checkpoint
    checkpoint_state_dict = checkpoint['state_dict'][f'model_{index}']
KeyError: 'model_0'
```
Sometimes the state_dict is already in the correct format here, so the model_{} keys don't exist.  I've altered this statement to check if that key is present or not first.
- With exp_manager.create_tensorboard_logger=false, adding tflops/sec as a metric raises an exception:
```
  File "/opt/NeMo/nemo/collections/common/metrics/perf_metrics.py", line 87, in on_train_end
    pl_module.logger.experiment.add_scalar("tflops_per_sec_per_gpu", tflops_per_sec_per_gpu)
AttributeError: 'NoneType' object has no attribute 'experiment'
```
I've added a condition to check if the logger exists before adding.

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
```

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# Additional Information
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