Add support for ellipsisMask in stridedSlice.

[annxingyuan]

StridedSlice with ellipsisMask is used by certain handwriting recognition models.

This fixes #2892

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This change is 

[nsthorat]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @annxingyuan, @dsmilkov, @nsthorat, and @pyu10055)

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tfjs-core/src/ops/slice_util.ts, line 86 at r1 (raw file):

// Creates full selection at the ellided dimensions. If the dimension matches
// the ellipsis mask, override the current start value. Otherwise, insert.
export function startIndicesWithEllidedDims(

nit: s/ellided/elided here and elsewhere

[pyu10055]

Reviewable status: complete! 2 of 1 approvals obtained (waiting on @annxingyuan, @dsmilkov, and @pyu10055)

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tfjs-core/src/ops/slice_util.ts, line 78 at r1 (raw file):

          ellipsisInsertionIndex, 0 /* num elements to delete */,
          1 /* element to add */);
      newStrides.pop();

I am curious why you need to pop one?

[annxingyuan]

Reviewable status: complete! 2 of 1 approvals obtained (waiting on @dsmilkov and @pyu10055)

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tfjs-core/src/ops/slice_util.ts, line 78 at r1 (raw file):

Previously, pyu10055 (Ping Yu) wrote…

I am curious why you need to pop one?

Wherever we insert the ellipsis, it pushes the explicit selections that come after the ellipsis (in case the explicit selection did not fill the rank of the input).

So if our explicit selection looks like this: [1:2, 2:3, 3:4] with ellipsisMask = 2 and the input rank is 5, then after normalization our selection will be turned to this: [1:2,:, 3:4, :, :]. However if we insert an ellipsis at position 1 then we need to push out the implicit full selection at positions 3 and 4 so the final selection looks like this: [1:2, :, :, :, 3:4].

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tfjs-core/src/ops/slice_util.ts, line 86 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

nit: s/ellided/elided here and elsewhere

Done