[BUG] Bug fix in SimpleImputer

[notaryanramani]

Reference Issues/PRs

Fixes a small bug in SimpleImputer #2347 .

What does this implement/fix? Explain your changes.

In my previous PR #2347 , I accidentally did not notice a bug. When strategy is a callable, it was fitting on 2d array instead of 1d array of independent case. I was about to correct it but the PR got merged. This PR fixes that bug and includes a test case for it.

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

Any other comments?

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I have added the following labels to this PR based on the title: [ $\color{#d73a4a}{\textsf{bug}}$ ]. I have added the following labels to this PR based on the changes made: [ $\color{#41A8F6}{\textsf{transformations}}$ ]. Feel free to change these if they do not properly represent the PR.

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