[slimtensor] Add utility functions to common_shims_slim

Add utility functions to the header-only common_shims_slim library:

1. DType constants:
   - `aoti_torch_dtype_float32()` - Returns 6 (ScalarType::Float)
   - `aoti_torch_dtype_bfloat16()` - Returns 15 (ScalarType::BFloat16)
   - `aoti_torch_dtype_int64()` - Returns 4 (ScalarType::Long)
   - `aoti_torch_dtype_int32()` - Returns 3 (ScalarType::Int)
   - `aoti_torch_dtype_int16()` - Returns 2 (ScalarType::Short)
   - `aoti_torch_dtype_int8()` - Returns 1 (ScalarType::Char)
   - `aoti_torch_dtype_bool()` - Returns 11 (ScalarType::Bool)

2. Device type constants:
   - `aoti_torch_device_type_cpu()` - Returns 0 (DeviceType::CPU)
   - `aoti_torch_device_type_cuda()` - Returns 1 (DeviceType::CUDA)

3. Grad mode functions (not supported in ExecuTorch):
   - `aoti_torch_grad_mode_is_enabled()` - Always returns false
   - `aoti_torch_grad_mode_set_enabled()` - Returns Ok for false, NotSupported for true

Differential Revision: [D90126250](https://our.internmc.facebook.com/intern/diff/D90126250/)

ghstack-source-id: 331923139
Pull Request resolved: #16457

Author: Gasoonjia

backends/aoti/common_shims_slim.h

@@ -288,6 +288,65 @@ inline AOTITorchError aoti_torch_get_device_index(
   return Error::Ok;
 }
 
+// ============================================================
+// DType Constants - These return PyTorch ScalarType enum values
+// ============================================================
+
+inline int32_t aoti_torch_dtype_float32() {
+  return 6; // ScalarType::Float
+}
+
+inline int32_t aoti_torch_dtype_bfloat16() {
+  return 15; // ScalarType::BFloat16
+}
+
+inline int32_t aoti_torch_dtype_int64() {
+  return 4; // ScalarType::Long
+}
+
+inline int32_t aoti_torch_dtype_int32() {
+  return 3; // ScalarType::Int
+}
+
+inline int32_t aoti_torch_dtype_int16() {
+  return 2; // ScalarType::Short
+}
+
+inline int32_t aoti_torch_dtype_int8() {
+  return 1; // ScalarType::Char
+}
+
+inline int32_t aoti_torch_dtype_bool() {
+  return 11; // ScalarType::Bool
+}
+
+// ============================================================
+// Device Type Constants
+// ============================================================
+
+inline int32_t aoti_torch_device_type_cpu() {
+  return 0; // DeviceType::CPU
+}
+
+inline int32_t aoti_torch_device_type_cuda() {
+  return 1; // DeviceType::CUDA
+}
+
+// ============================================================
+// Grad Mode Functions (not supported in ExecuTorch)
+// ============================================================
+
+inline bool aoti_torch_grad_mode_is_enabled() {
+  return false; // ExecuTorch doesn't support autograd
+}
+
+inline AOTITorchError aoti_torch_grad_mode_set_enabled(bool enabled) {
+  if (enabled) {
+    return Error::NotSupported; // Grad mode not supported in ExecuTorch
+  }
+  return Error::Ok;
+}
+
 } // namespace aoti
 } // namespace backends
 } // namespace executorch

backends/aoti/tests/test_common_shims_slim.cpp

@@ -589,3 +589,44 @@ TEST_F(CommonShimsSlimTest, ConsistentPointerReturn) {
   EXPECT_EQ(aoti_torch_get_strides(tensor, &strides_ptr2), Error::Ok);
   EXPECT_EQ(strides_ptr1, strides_ptr2);
 }
+
+// ============================================================================
+// DType Constants Tests
+// ============================================================================
+
+TEST_F(CommonShimsSlimTest, DTypeConstants) {
+  // Verify dtype constants match expected PyTorch ScalarType values
+  EXPECT_EQ(aoti_torch_dtype_float32(), 6); // ScalarType::Float
+  EXPECT_EQ(aoti_torch_dtype_bfloat16(), 15); // ScalarType::BFloat16
+  EXPECT_EQ(aoti_torch_dtype_int64(), 4); // ScalarType::Long
+  EXPECT_EQ(aoti_torch_dtype_int32(), 3); // ScalarType::Int
+  EXPECT_EQ(aoti_torch_dtype_int16(), 2); // ScalarType::Short
+  EXPECT_EQ(aoti_torch_dtype_int8(), 1); // ScalarType::Char
+  EXPECT_EQ(aoti_torch_dtype_bool(), 11); // ScalarType::Bool
+}
+
+// ============================================================================
+// Device Type Constants Tests
+// ============================================================================
+
+TEST_F(CommonShimsSlimTest, DeviceTypeConstants) {
+  EXPECT_EQ(aoti_torch_device_type_cpu(), 0); // DeviceType::CPU
+  EXPECT_EQ(aoti_torch_device_type_cuda(), 1); // DeviceType::CUDA
+}
+
+// ============================================================================
+// Grad Mode Tests
+// ============================================================================
+
+TEST_F(CommonShimsSlimTest, GradModeIsEnabled) {
+  // ExecuTorch doesn't support autograd, so should always return false
+  EXPECT_EQ(aoti_torch_grad_mode_is_enabled(), false);
+}
+
+TEST_F(CommonShimsSlimTest, GradModeSetEnabled) {
+  // Setting to false should succeed
+  EXPECT_EQ(aoti_torch_grad_mode_set_enabled(false), Error::Ok);
+
+  // Setting to true should fail (not supported in ExecuTorch)
+  EXPECT_EQ(aoti_torch_grad_mode_set_enabled(true), Error::NotSupported);
+}