[MLIR][Tensor] Remove assert in PadOp builder

The assert is misplaced as the result type is allowed to be null. A few
lines below the result type is inferred if it is passed a nullptr.
Besides, this behavior is described in the documentation of the builder.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D136262

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

@@ -2407,7 +2407,6 @@ void PadOp::build(OpBuilder &b, OperationState &result, Type resultType,
                   Value source, ArrayRef<OpFoldResult> low,
                   ArrayRef<OpFoldResult> high, bool nofold,
                   ArrayRef<NamedAttribute> attrs) {
-  assert(resultType.isa<RankedTensorType>());
   auto sourceType = source.getType().cast<RankedTensorType>();
   SmallVector<Value, 4> dynamicLow, dynamicHigh;
   SmallVector<int64_t, 4> staticLow, staticHigh;