webnn: Support gatherElements in tflite converter

There is no TFLite operator to map directly WebNN gatherElements, but
it can be supported with tfl.gather_nd, the test cases[1] and the
doc[2] show the gather_nd can gether not only slices but also elements,
but gather_nd can't gather along the axis, so the indices used by
gather_nd need to be calculated by unravelling the flat index and
adjusting value with the axis.

The coordinates are calculated by the stride of each dimension. For
example a shape (3, 2) array can map to the following coordinates:
//         index                      row, col
//     [[0,  1,]                  [[0, 0], [0, 1],
//      [2,  3,]      =>            [1, 0], [1, 1]
//      [4,  5,]]                   [2, 0], [2, 1]]

Adjusting the coordinates with WebNN indices operand along the axis,
//   unravelled index   WebNN indices   axis = 0       TFLite indices
//  [[0, 0], [0, 1],     [[1, 0],                  [[1 ,0], [0, 1],
//   [1, 0], [1, 1]       [2, 1],         =>        [2, 0], [1, 1],
//   [2, 0], [2, 1]]      [0, 2]]                   [0, 0], [2, 1]]

[1] https://www.tensorflow.org/mlir/tfl_ops#tflgather_nd_tflgatherndop
[2] https://www.tensorflow.org/guide/tensor_slicing#insert_data_into_tensors

Bug: 40206287
Change-Id: I61ad75585405039da10af05f3300799fe8f32855
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6000111
Reviewed-by: ningxin hu <[email protected]>
Reviewed-by: Reilly Grant <[email protected]>
Commit-Queue: Junwei Fu <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1387213}

Author: fujunwei

webnn/conformance_tests/gatherElements.https.any.js

@@ -99,6 +99,80 @@ const gatherElementsTests = [
       }
     }
   },
+  ,
+  {
+    'name': 'gatherElements float32 2D input and int32 indices options.axis=0',
+    'graph': {
+      'inputs': {
+        'gatherElementsInput': {
+          'data': [
+            -66.05901336669922, -68.9197006225586, -77.02045440673828,
+            -26.158037185668945, 89.0337142944336, -45.89653396606445,
+            43.84803771972656, 48.81806945800781, 51.79948425292969
+          ],
+          'descriptor': {shape: [3, 3], dataType: 'float32'}
+        },
+        'gatherElementsIndices': {
+          'data': [1, 0, 2, 2, 1, 0],
+          'descriptor': {shape: [2, 3], dataType: 'int32'},
+          'constant': true
+        }
+      },
+      'operators': [{
+        'name': 'gatherElements',
+        'arguments': [
+          {'input': 'gatherElementsInput'},
+          {'indices': 'gatherElementsIndices'}, {'options': {'axis': 0}}
+        ],
+        'outputs': 'gatherElementsOutput'
+      }],
+      'expectedOutputs': {
+        'gatherElementsOutput': {
+          'data': [
+            -26.158037185668945, -68.9197006225586, 51.79948425292969,
+            43.84803771972656, 89.0337142944336, -77.02045440673828
+          ],
+          'descriptor': {shape: [2, 3], dataType: 'float32'}
+        }
+      }
+    }
+  },
+  {
+    'name': 'gatherElements float32 3D input and int32 indices options.axis=0',
+    'graph': {
+      'inputs': {
+        'gatherElementsInput': {
+          'data': [
+            -66.05901336669922, -68.9197006225586, -77.02045440673828,
+            -26.158037185668945, 89.0337142944336, -45.89653396606445,
+            43.84803771972656, 48.81806945800781
+          ],
+          'descriptor': {shape: [2, 2, 2], dataType: 'float32'}
+        },
+        'gatherElementsIndices': {
+          'data': [1, 0, 0, 1],
+          'descriptor': {shape: [1, 2, 2], dataType: 'int32'},
+          'constant': true
+        }
+      },
+      'operators': [{
+        'name': 'gatherElements',
+        'arguments': [
+          {'input': 'gatherElementsInput'}, {'indices': 'gatherElementsIndices'}
+        ],
+        'outputs': 'gatherElementsOutput'
+      }],
+      'expectedOutputs': {
+        'gatherElementsOutput': {
+          'data': [
+            89.0337142944336, -68.9197006225586, -77.02045440673828,
+            48.81806945800781
+          ],
+          'descriptor': {shape: [1, 2, 2], dataType: 'float32'}
+        }
+      }
+    }
+  },
   {
     'name': 'gatherElements float32 3D input and int32 negative indices',
     'graph': {