Make example input and PyFuncInput support csc csr matrix

[WeichenXu123]

What changes are proposed in this pull request?

Make example input and PyFuncInput support csc csr matrix

How is this patch tested?

Unit tests.

Release Notes

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Components

• area/artifacts: Artifact stores and artifact logging
• area/build: Build and test infrastructure for MLflow
• area/docs: MLflow documentation pages
• area/examples: Example code
• area/model-registry: Model Registry service, APIs, and the fluent client calls for Model Registry
• area/models: MLmodel format, model serialization/deserialization, flavors
• area/projects: MLproject format, project running backends
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• area/server-infra: MLflow Tracking server backend
• area/tracking: Tracking Service, tracking client APIs, autologging

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• area/uiux: Front-end, user experience, plotting, JavaScript, JavaScript dev server
• area/docker: Docker use across MLflow's components, such as MLflow Projects and MLflow Models
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• area/windows: Windows support

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• rn/bug-fix - A user-facing bug fix worth mentioning in the release notes
• rn/documentation - A user-facing documentation change worth mentioning in the release notes

[tomasatdatabricks]

Looks good to me in general. One aspect I am not sure if I understand is - do we store the example as dense matrix? I think we should have a way to store it as sparse matrix and load it back as sparse matrix.

[tomasatdatabricks]

1. can we do elif isinstance(input_tesnor,csr_matrix, csc_matrix)?
2. does this mean we store sparse input as dense vector? or is this stored as array of indices and array of values?

[WeichenXu123]

I update code to store sparse input as data/indices/indptr vectors.
I also split csc/csr saving code out of the ndarray saving code.

[tomasatdatabricks]

Thanks for the changes! The looks good in great.

I left a minor comment about the example type string for sparse matrices but it's only a suggestion. I am also ok with it as is.