Support len in ChemicalSymbolsToInts, len of it returns number of supported elements (#286)

* Support len in ChemicalSymbolsToInts

* fix

* flake8

tests/test_utils.py

+import unittest
+import torchani
+
+
+class TestUtils(unittest.TestCase):
+
+    def testChemicalSymbolsToInts(self):
+        str2i = torchani.utils.ChemicalSymbolsToInts('ABCDEF')
+        self.assertEqual(len(str2i), 6)
+        self.assertListEqual(str2i('BACCC').tolist(), [1, 0, 2, 2, 2])
+
+
+if __name__ == '__main__':
+    unittest.main()

torchani/utils.py

@@ -226,15 +226,16 @@ class ChemicalSymbolsToInts:
     """
 
     def __init__(self, all_species):
-        self.rev_species = {}
-        for i, s in enumerate(all_species):
-            self.rev_species[s] = i
+        self.rev_species = {s: i for i, s in enumerate(all_species)}
 
     def __call__(self, species):
         """Convert species from squence of strings to 1D tensor"""
         rev = [self.rev_species[s] for s in species]
         return torch.tensor(rev, dtype=torch.long)
 
+    def __len__(self):
+        return len(self.rev_species)
+
 
 def hessian(coordinates, energies=None, forces=None):
     """Compute analytical hessian from the energy graph or force graph.