[ENH] Extend KernelMixture to support 2D per-instance weights

[patelchaitany]

Reference Issues/PRs

Fixes #836

What does this implement/fix? Explain your changes.

Extended KernelMixture to accept a 2D weights array of shape (n_instances, n_support), where each row provides a separate weight vector for each distribution instance. Previously, only 1D weights (shared across all instances) were supported.

Does your contribution introduce a new dependency? If yes, which one?

No

What should a reviewer concentrate their feedback on?

Did you add any tests for the change?

No

Any other comments?

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[fkiraly]

Looks good!