Adds per-axis bin counts feature in GridPartition

[dhruvDev23]

This PR adds the feature mentioned by @FarnazH in this comment.

Currently all axes in GridPartition are forced to use name number of bins, This feature allow users to specify different number of bins for each feature dimension.

Tests: Adds 4 new test functions in test_partition.py covering:

1. Initialization: Verifies list and int storage in nbins_axis.
2. Execution: Verifies all 4 partitioning methods work correctly with per-axis counts.
3. Error Handling: Verifies ValueError is raised for mismatched list lengths.
4. Backward Compatibility: Verifies that nbins_axis=5 produces identical results to nbins_axis=[5, 5].