[Bugfix] Fix a bug in filtering checkpoints
# What does this PR do ?

When we cannot find `self.monitor` in `checkpoint`, `checkpoint.find(self.monitor)` should be -1 so `index` should be `len(self.monitor)` according to L98.

With the current code, checkpoints such as `mp_rank_00_model_weights.ckpt` would go to the True condition and cause an error in Line 103 because  it cannot find the correct `value` to be converted to float.
https://github.com/NVIDIA/NeMo/blob/5c3ed943d8b23ae32719f8fd0e51fca5801596d3/nemo/utils/callbacks/nemo_model_checkpoint.py#L98-L103.

**Collection**: [Note which collection this PR will affect]

- NeMo/nemo/utils/callbacks/nemo_model_checkpoint.py

# Changelog 


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# Additional Information
* Should fix https://github.com/NVIDIA/NeMo/issues/6796
