Fix torch.jit.ScriptModule.zero_grad.

[hiyuh]

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.

Note;

• intentionally, omit RELEASENOTES.md update ATM.
  • due to avoid multiple conflict&rebase annoyance while MR review.
  • i'll update later, before merging this MR, if upstream prefers.
• i'm not sure whether foreach loop of ScriptModule.zero_grad in src/TorchSharp/JIT/ScriptModule.cs is actually needed.
  • this is just mimicking what Module.zero_grad in src/TorchSharp/NN/Module.cs does.