CountFeatureSelectingEstimator no selection support

[Lynx1820]

Issue: The GatherElements ONNX operator used by SlotsDroppingTransformer (transformer for CountFeatureSelectingEstimator) has issues when there's nothing selected, so mlnet needs to handle this case separately.

This PR

• Fixes the issue mentioned above
• Modifies the estimator's test to be more robust

Other issues:

• There's no support for string initializing in ONNX, which prevents us from handling the case where no string columns get selected.
• The documentation for CountFeatureSelectingEstimator is ambiguous. It states that it supports input as "vector or scalar of numeric, text or key data types". However, only strings, doubles, and singles are supported.

[frank-dong-ms-zz]

_srcTypes[iinfo] is VectorDataViewType && slots.Count() > 0 [](start = 20, length = 59)

what if _srcTypes[iinfo] is VectorDataViewType && slots.Count() == 0? should we return true or false?
in current implementation seems it will goes into the "else" condition in line 917 and which should handling double numeric data type #Resolved

[Lynx1820]

If slots.Count == 0, then the column get's "suppressed". It's not handled well by the GatherElements operator, so we create an empty vector in the else clause. Does that make sense?

[frank-dong-ms-zz]

yes, but we better to make the logic clear like below, don't let different cases fall into same condition which will make code harder to read and will cause potential issue in future:

if (_srcTypes[iinfo] is VectorDataViewType)
{
if (count > 0) { ... }
else { ... }
}
else { ... }

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In reply to: 404404988 [](ancestors = 404404988)

[Lynx1820]

fixed.

[frank-dong-ms-zz]