[mlir][tensor] Fix getReassociationForCollapse for tensor/scalar reshapes

Commit 6e5a142 changed the behavior of the function when computing
reassociations between tensors (consisting of unit/dynamic dimensions) and
scalars/0d vectors. The IR representation for such reshapes actually expects
an empty reassociation, like so:
```
func.func @example(%arg0 : tensor<?x?x?xf32>) -> tensor<f32> {
  %0 = tensor.collapse_shape %arg0 [] : tensor<?x?x?xf32> into tensor<f32>
}
```

Restore the original behavior - the routine should resort to reporting
failures when compile time-known non-unit dimensions are part of the
attempted reassociation.

Signed-off-by: Artem Gindinson <[email protected]>

Author: AGindinson

mlir/lib/Dialect/Utils/ReshapeOpsUtils.cpp

@@ -299,19 +299,17 @@ mlir::getReassociationIndicesForCollapse(ArrayRef<int64_t> sourceShape,
   // this utility).
   if (numSourceDims <= numTargetDims)
     return std::nullopt;
-  // Early handling for scalar target types.
+  // Early handling for scalar target types. We should report an invalid
+  // reassociation for non-unit static dimensions - no chance to collapse these
+  // into a scalar.
   if (numTargetDims == 0) {
-    ReassociationIndices allSourceIndices;
-    allSourceIndices.reserve(numSourceDims);
     for (unsigned sourceDimIdx = 0; sourceDimIdx < numSourceDims;
          ++sourceDimIdx) {
       int64_t sourceSize = sourceShape[sourceDimIdx];
-      // All source dimensions must be unit or dynamic.
       if (sourceSize != 1 && sourceSize != ShapedType::kDynamic)
         return std::nullopt;
-      allSourceIndices.push_back(sourceDimIdx);
     }
-    return SmallVector<ReassociationIndices>{allSourceIndices};
+    return SmallVector<ReassociationIndices>{};
   }
 
   // Collect source ranges by iterating over the target shape left-to-right.

mlir/unittests/Dialect/Utils/ReshapeOpsUtilsTest.cpp

@@ -23,16 +23,16 @@ makeOptionalIndices(std::initializer_list<ReassociationIndices> list) {
 
 TEST(ReassociationIndicesForCollapse, ScalarTest) {
   EXPECT_EQ(getReassociationIndicesForCollapse({1}, {}),
-            makeOptionalIndices({{0}}));
+            makeOptionalIndices({}));
   EXPECT_EQ(getReassociationIndicesForCollapse({1, 1}, {}),
-            makeOptionalIndices({{0, 1}}));
+            makeOptionalIndices({}));
   EXPECT_EQ(getReassociationIndicesForCollapse({ShapedType::kDynamic}, {}),
-            makeOptionalIndices({{0}}));
+            makeOptionalIndices({}));
   EXPECT_EQ(getReassociationIndicesForCollapse({1, ShapedType::kDynamic,
                                                 ShapedType::kDynamic, 1,
                                                 ShapedType::kDynamic},
                                                {}),
-            makeOptionalIndices({{0, 1, 2, 3, 4}}));
+            makeOptionalIndices({}));
 }
 
 TEST(ReassociationIndicesForCollapse, ScalarTestFailure) {