Speed up major_allele_frequency calculation using a built-in dictionary instead of a Counter class. Using a plain dictionary is ~5x faster when tested with 10,000 variants, 316 calls per variant.
Speed up major_allele_frequency calculation using a built-in dictionary instead of a Counter class. Using a plain dictionary is ~5x faster when tested with 10,000 variants, 316 calls per variant.

PiperOrigin-RevId: 240995523

google/deepvariant,google/deepvariant,google/deepvariant