Fix torch.jit.ScriptModule.zero_grad.

TorchSharp 0.105.0 doesn't have torch.jit.ScriptModule.zero_grad and
falls back into torch.nn.Module.zero_grad incorrectly, then terminates
silently.
Most probably, because JITModule is not compatible to NNModule in
LibTorchSharp.

And as reported in pytorch/pytorch#27144,
libtorch also doesn't have torch::jit::Module::zero_grad.
As a workaround, manually loop over the parameters and zero them out
like optimizer does.

Author: Masaru Kimura

src/Native/LibTorchSharp/THSJIT.cpp

@@ -68,6 +68,23 @@ int THSJIT_Module_is_training(JITModule module)
     return (*module)->is_training();
 }
 
+void THSJIT_Module_zero_grad(const JITModule module, bool set_to_none)
+{
+    // According to https://github.com/pytorch/pytorch/issues/27144,
+    // torch::jit::Module has no zero_grad().
+    // As a workaround, manually loop over the parameters and zero them out like optimizer does;
+    // https://github.com/pytorch/pytorch/blob/v2.5.1/torch/csrc/api/src/optim/optimizer.cpp#L123
+    for (auto& p : (*module)->parameters()) {
+        if (p.mutable_grad().defined()) {
+            p.mutable_grad().detach_();
+            if (set_to_none)
+                p.mutable_grad().reset();
+            else
+                p.mutable_grad().zero_();
+        }
+    }
+}
+
 void THSJIT_Module_train(JITModule module, bool on)
 {
     (*module)->train(on);

src/Native/LibTorchSharp/THSJIT.h

@@ -44,6 +44,7 @@ EXPORT_API(void) THSJIT_Module_invoke(const JITModule module, const char* name,
 EXPORT_API(void) THSJIT_CompilationUnit_Invoke(const JITCompilationUnit module, const char* method, const TensorOrScalar* tensorPtrs, const int length, TensorOrScalar* (*allocator)(int32_t idx, size_t length), int8_t* typeCode, int32_t idx);
 
 EXPORT_API(int) THSJIT_Module_is_training(JITModule module);
+EXPORT_API(void) THSJIT_Module_zero_grad(const JITModule module, bool set_to_none);
 EXPORT_API(void) THSJIT_Module_train(JITModule module, bool on);
 EXPORT_API(void) THSJIT_Module_eval(JITModule module);
 

src/TorchSharp/JIT/ScriptModule.cs

@@ -143,6 +143,23 @@ public override bool training {
                     }
                 }
 
+                public override void zero_grad(bool set_to_none = true)
+                {
+                    THSJIT_Module_zero_grad(handle, set_to_none);
+                    CheckForErrors();
+
+                    foreach (var (_, p) in named_parameters()) {
+                        using var grad = p.grad;
+                        if (grad is not null) {
+                            if (set_to_none) {
+                                p.grad = null;
+                            } else {
+                                grad.zero_();
+                            }
+                        }
+                    }
+                }
+
                 protected internal override nn.Module _to(Device device, ScalarType dtype, bool non_blocking)
                 {
                     if (device.type != DeviceType.CUDA) { device = new Device(device.type, -1); };

src/TorchSharp/PInvoke/LibTorchSharp.THSJIT.cs

@@ -57,6 +57,9 @@ internal static partial class NativeMethods
         [return: MarshalAs(UnmanagedType.U1)]
         internal static extern bool THSJIT_Module_is_training(torch.nn.Module.HType module);
 
+        [DllImport("LibTorchSharp")]
+        internal static extern void THSJIT_Module_zero_grad(torch.nn.Module.HType module, [MarshalAs(UnmanagedType.U1)] bool set_to_none);
+
         [DllImport("LibTorchSharp")]
         internal static extern void THSJIT_Module_to_device(torch.nn.Module.HType module, long deviceType, long deviceIndex);