improve test coverage in ML Model utilities

[s-g-1]

Describe the solution you'd like rainfall_predictor/dataset.py and rainfall_predictor/utils.py need all of their functions covered by tests.

[s-g-1]

Asat PR-18, in dataset, the following functions remain untested

• gen_sequence,
• gen_labels,
• groupdf,

in utils, all functions have a test at 97% coverage for the module