Add an `ignore_index` parameter to the metric factory

[AjeyPaiK]

Is your feature request related to a problem? Please describe.
Currently, when we run training using ahcore for segmentation, we don't have an option to ignore some unwanted classes while computing metrics.

Describe the solution you'd like
A flag indicating the class index to be ignored while computing the metrics.

Describe alternatives you've considered
N/A

Additional context
N/A