[ET Device Support] DeviceAllocator interface and DeviceAllocatorRegistry

Pull Request resolved: #17535

This diff introduces the `DeviceAllocator` abstract interface and `DeviceAllocatorRegistry` for device-specific memory allocation. This is a foundational abstraction that enables the runtime to dispatch memory operations to the appropriate device backend other than CPU (CUDA, etc.).

**DeviceAllocator interface provides:**
- `allocate()` / `deallocate()` - Dynamic device memory allocation
- `copy_host_to_device()` / `copy_device_to_host()` - Data transfer between host and device
- `device_type()` - Returns the device type this allocator handles

**DeviceAllocatorRegistry provides:**
- Singleton registry mapping DeviceType → DeviceAllocator
- `register_allocator()` / `get_allocator()` methods
- Fixed-size array indexed by device type (no dynamic allocation, embedded-friendly)

**Design notes:**
- Registry stores raw pointers (non-owning) - allocators are expected to be singletons with static lifetime
- Follows ExecuTorch's embedded-first philosophy (no std::unique_ptr, no heap allocation in registry)
- Convenience free functions `register_device_allocator()` and `get_device_allocator()` for ease of use
ghstack-source-id: 354740821

Differential Revision: [D93635656](https://our.internmc.facebook.com/intern/diff/D93635656/)

Author: Gasoonjia

runtime/core/device_allocator.cpp

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <executorch/runtime/core/device_allocator.h>
+
+#include <executorch/runtime/platform/assert.h>
+
+namespace executorch {
+namespace runtime {
+
+DeviceAllocatorRegistry& DeviceAllocatorRegistry::instance() {
+  static DeviceAllocatorRegistry registry;
+  return registry;
+}
+
+void DeviceAllocatorRegistry::register_allocator(
+    etensor::DeviceType type,
+    DeviceAllocator* alloc) {
+  auto index = static_cast<size_t>(type);
+  ET_CHECK_MSG(
+      index < etensor::kNumDeviceTypes,
+      "Invalid device type: %d",
+      static_cast<int>(type));
+  ET_CHECK_MSG(
+      allocators_[index] == nullptr,
+      "Allocator already registered for device type: %d",
+      static_cast<int>(type));
+  allocators_[index] = alloc;
+}
+
+DeviceAllocator* DeviceAllocatorRegistry::get_allocator(
+    etensor::DeviceType type) {
+  auto index = static_cast<size_t>(type);
+  if (index >= etensor::kNumDeviceTypes) {
+    return nullptr;
+  }
+  return allocators_[index];
+}
+
+// Convenience free functions
+
+void register_device_allocator(
+    etensor::DeviceType type,
+    DeviceAllocator* alloc) {
+  DeviceAllocatorRegistry::instance().register_allocator(type, alloc);
+}
+
+DeviceAllocator* get_device_allocator(etensor::DeviceType type) {
+  return DeviceAllocatorRegistry::instance().get_allocator(type);
+}
+
+} // namespace runtime
+} // namespace executorch

runtime/core/device_allocator.h

@@ -0,0 +1,156 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <cstddef>
+
+#include <executorch/runtime/core/error.h>
+#include <executorch/runtime/core/portable_type/device.h>
+#include <executorch/runtime/core/result.h>
+
+namespace executorch {
+namespace runtime {
+
+/**
+ * Abstract interface for device-specific memory allocation.
+ *
+ * Each device type (CUDA, etc.) provides a concrete implementation
+ * that handles memory allocation on that device. Implementations are
+ * expected to be singletons with static lifetime, registered via
+ * DeviceAllocatorRegistry.
+
+ */
+class DeviceAllocator {
+ public:
+  virtual ~DeviceAllocator() = default;
+  /**
+   * Allocate device memory.
+   *
+   * @param nbytes Number of bytes to allocate.
+   * @param index The device index.
+   * @return A Result containing the device pointer on success, or an error.
+   */
+  virtual Result<void*> allocate(size_t nbytes, etensor::DeviceIndex index) = 0;
+
+  /**
+   * Deallocate device memory previously allocated via allocate().
+   *
+   * @param ptr Pointer to the memory to deallocate.
+   * @param index The device index.
+   */
+  virtual void deallocate(void* ptr, etensor::DeviceIndex index) = 0;
+
+  /**
+   * Copy data from host memory to device memory.
+   *
+   * @param dst Destination pointer (device memory).
+   * @param src Source pointer (host memory).
+   * @param nbytes Number of bytes to copy.
+   * @param index The device index.
+   * @return Error::Ok on success, or an appropriate error code on failure.
+   */
+  virtual Error copy_host_to_device(
+      void* dst,
+      const void* src,
+      size_t nbytes,
+      etensor::DeviceIndex index) = 0;
+
+  /**
+   * Copy data from device memory to host memory.
+   *
+   * @param dst Destination pointer (host memory).
+   * @param src Source pointer (device memory).
+   * @param nbytes Number of bytes to copy.
+   * @param index The device index.
+   * @return Error::Ok on success, or an appropriate error code on failure.
+   */
+  virtual Error copy_device_to_host(
+      void* dst,
+      const void* src,
+      size_t nbytes,
+      etensor::DeviceIndex index) = 0;
+
+  /**
+   * Returns the device type this allocator handles.
+   */
+  virtual etensor::DeviceType device_type() const = 0;
+};
+
+/**
+ * Registry for device allocators.
+ *
+ * Provides a global mapping from DeviceType to DeviceAllocator instances.
+ * Device allocators register themselves at static initialization time,
+ * and the runtime queries the registry to find the appropriate allocator
+ * for a given device type.
+ */
+class DeviceAllocatorRegistry {
+ public:
+  /**
+   * Returns the singleton instance of the registry.
+   */
+  static DeviceAllocatorRegistry& instance();
+
+  /**
+   * Register an allocator for a specific device type.
+   *
+   * @param type The device type this allocator handles.
+   * @param alloc Pointer to the allocator (must have static lifetime).
+   */
+  void register_allocator(etensor::DeviceType type, DeviceAllocator* alloc);
+
+  /**
+   * Get the allocator for a specific device type.
+   *
+   * @param type The device type.
+   * @return Pointer to the allocator, or nullptr if not registered.
+   */
+  DeviceAllocator* get_allocator(etensor::DeviceType type);
+
+ private:
+  DeviceAllocatorRegistry() = default;
+
+  // Fixed-size array indexed by device type. This avoids dynamic allocation
+  // and is suitable for embedded environments.
+  DeviceAllocator* allocators_[etensor::kNumDeviceTypes] = {};
+};
+
+// Convenience free functions
+
+/**
+ * Register a device allocator for a specific device type.
+ *
+ * @param type The device type this allocator handles.
+ * @param alloc Pointer to the allocator (must have static lifetime).
+ */
+void register_device_allocator(
+    etensor::DeviceType type,
+    DeviceAllocator* alloc);
+
+/**
+ * Get the device allocator for a specific device type.
+ *
+ * @param type The device type.
+ * @return Pointer to the allocator, or nullptr if not registered.
+ */
+DeviceAllocator* get_device_allocator(etensor::DeviceType type);
+
+} // namespace runtime
+} // namespace executorch
+
+namespace torch {
+namespace executor {
+// TODO(T197294990): Remove these deprecated aliases once all users have moved
+// to the new `::executorch` namespaces.
+using ::executorch::runtime::DeviceAllocator;
+using ::executorch::runtime::DeviceAllocatorRegistry;
+using ::executorch::runtime::get_device_allocator;
+using ::executorch::runtime::register_device_allocator;
+} // namespace executor
+} // namespace torch