[torch] Fix unsqueezed output shape in canonicalization of AtenUnflattenIntOp (#3730)

Fixes https://github.com/iree-org/iree/issues/18562.

During canonicalization pass on `AtenUnflattenIntOp`, if the second dim
was statically equal to one, we would create an `AtenAddIntOp` to add
one to the dimension obtained from `op.getDim()`. This, when passed into
`Torch::unsqueezeTensor()`, would make it get interpreted as
non-constant, which would lead to MLIR failing an assertion when
`UnsqueezeOp` would later get lowered into `ExpandShapeOp`, as the
output of the `UnsqueezeOp` would consist of only dynamic dims.

This patch fixes this behavior, by extracting the integer value from the
dim if it was constant, and then emitting a `ConstantIntOp` from
(dim+1). This creates an output with static shape.

lib/Dialect/Torch/IR/TorchOps.cpp

@@ -2189,6 +2189,9 @@ void AtenUnflattenIntOp::getCanonicalizationPatterns(
     if (dim0 != 1 && dim1 != 1)
       return failure();
     Value unflattenDim = op.getDim();
+    int64_t dimAsInt;
+    bool dimWasConstant =
+        matchPattern(unflattenDim, m_TorchConstantInt(&dimAsInt));
     Value self = op.getSelf();
     Value cstMOne = rewriter.create<Torch::ConstantIntOp>(op.getLoc(), -1);
     // the runtime asserts below are introduced to catch malformed unflatten ops
@@ -2217,9 +2220,22 @@ void AtenUnflattenIntOp::getCanonicalizationPatterns(
     }
     if (dim1 == 1) {
       // unsqueeze at dim + 1
-      Value cstOne = rewriter.create<Torch::ConstantIntOp>(op.getLoc(), 1);
-      Value dimPlusOne =
-          rewriter.create<AtenAddIntOp>(op.getLoc(), unflattenDim, cstOne);
+      Value dimPlusOne;
+      if (!dimWasConstant) {
+        Value cstOne = rewriter.create<Torch::ConstantIntOp>(op.getLoc(), 1);
+        dimPlusOne =
+            rewriter.create<AtenAddIntOp>(op.getLoc(), unflattenDim, cstOne);
+      } else {
+        // If dim was constant, creating an AtenAddIntOp will make
+        // Torch::unsqueezeTensor() interpret it as still not being a constant,
+        // and the resultant shape would consist of only dynamic dims. To fix
+        // this, emit a ConstantIntOp for (dim + 1) to avoid an assertion
+        // failure, when AtenUnsqueezeOp is in a later pass converted to
+        // ExpandShapeOp, which is bound to fail shape inference in MLIR if
+        // output dims are dynamic.
+        dimPlusOne = rewriter.create<Torch::ConstantIntOp>(
+            op.getLoc(), rewriter.getI64IntegerAttr(dimAsInt + 1));
+      }
       FailureOr<Value> maybeUnsqueeze =
           Torch::unsqueezeTensor(rewriter, op, self, dimPlusOne);
       if (failed(maybeUnsqueeze))