code: def group_reads_by_allele(allele_reads):
      
      allele_to_reads_dict = defaultdict(list)
      for allele_read in allele_reads:
          allele_to_reads_dict[allele_read.allele].append(allele_read)
      return allele_to_reads_dict
docs: Returns dictionary mapping each allele's nucleotide sequence to a list of
      supporting AlleleRead objects.