Add example for using TorchScript (#328)

docs/index.rst

@@ -16,6 +16,7 @@ Welcome to TorchANI's documentation!
 
     examples/energy_force
     examples/ase_interface
+    examples/jit
     examples/vibration_analysis
     examples/load_from_neurochem
     examples/nnp_training

examples/jit.py

+# -*- coding: utf-8 -*-
+"""
+Using TorchScript to serialize and deploy model
+===============================================
+
+Models in TorchANI's model zoo support TorchScript. TorchScript is a way to create
+serializable and optimizable models from PyTorch code. It allows users to saved their
+models from a Python process and loaded in a process where there is no Python dependency.
+"""
+
+###############################################################################
+# To begin with, let's first import the modules we will use:
+import torch
+import torchani
+
+###############################################################################
+# Let's now load the built-in ANI-1ccx models. The builtin ANI-1ccx contains 8
+# models trained with diffrent initialization.
+model = torchani.models.ANI1ccx()
+
+###############################################################################
+# It is very easy to compile and save the model using `torch.jit`.
+compiled_model = torch.jit.script(model)
+torch.jit.save(compiled_model, 'compiled_model.pt')
+
+###############################################################################
+# Besides compiling the ensemble, it is also possible to compile a single network
+compiled_model0 = torch.jit.script(model[0])
+torch.jit.save(compiled_model0, 'compiled_model0.pt')
+
+###############################################################################
+# For testing purposes, we will now load the models we just saved and see if they
+# produces the same output as the original model:
+loaded_compiled_model = torch.jit.load('compiled_model.pt')
+loaded_compiled_model0 = torch.jit.load('compiled_model0.pt')
+
+
+###############################################################################
+# We use the molecule below to test:
+coordinates = torch.tensor([[[0.03192167, 0.00638559, 0.01301679],
+                             [-0.83140486, 0.39370209, -0.26395324],
+                             [-0.66518241, -0.84461308, 0.20759389],
+                             [0.45554739, 0.54289633, 0.81170881],
+                             [0.66091919, -0.16799635, -0.91037834]]])
+species = model.species_to_tensor('CHHHH').unsqueeze(0)
+
+###############################################################################
+# And here is the result:
+_, energies_ensemble = model((species, coordinates))
+_, energies_single = model[0]((species, coordinates))
+_, energies_ensemble_jit = loaded_compiled_model((species, coordinates))
+_, energies_single_jit = loaded_compiled_model0((species, coordinates))
+print('Ensemble energy, eager mode vs loaded jit:', energies_ensemble.item(), energies_ensemble_jit.item())
+print('Single network energy, eager mode vs loaded jit:', energies_single.item(), energies_single_jit.item())