[SPARK-29867][ML][PYTHON] Add __repr__ in Python ML Models

[huaxingao]

What changes were proposed in this pull request?

Add __repr__ in Python ML Models

Why are the changes needed?

In Python ML Models, some of them have __repr__, others don't. In the doctest, when calling Model.setXXX, some of the Models print out the xxxModel... correctly, some of them can't because of lacking the __repr__ method. For example:

    >>> gm = GaussianMixture(k=3, tol=0.0001, seed=10)
    >>> model = gm.fit(df)
    >>> model.setPredictionCol("newPrediction")
    GaussianMixture...

After the change, the above code will become the following:

    >>> gm = GaussianMixture(k=3, tol=0.0001, seed=10)
    >>> model = gm.fit(df)
    >>> model.setPredictionCol("newPrediction")
    GaussianMixtureModel...

Does this PR introduce any user-facing change?

Yes.

How was this patch tested?

doctest

[SparkQA]

Test build #113651 has finished for PR 26489 at commit a6c649a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[zhengruifeng]

Can we put this method to some common class (like JavaModel) and add __repr__ only in classes not extending JavaModel(like OneVsRestModel) ?

[SparkQA]

Test build #113797 has finished for PR 26489 at commit c9b5e28.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #113799 has finished for PR 26489 at commit 6d97563.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Yep, good change. Looks OK pending tests.

[dongjoon-hyun]

+1, LGTM. The common __repl__ is nice.
Thank you, @huaxingao , @zhengruifeng , @srowen .
Merged to master.

[huaxingao]

Thanks! @dongjoon-hyun @srowen @zhengruifeng