Support custom exception message (#41907)

Summary:
Raise and assert used to have a hard-coded error message "Exception". User provided error message was ignored. This PR adds support to represent user's error message in TorchScript.

This breaks backward compatibility because now we actually need to script the user's error message, which can potentially contain unscriptable expressions. Such programs can break when scripting, but saved models can still continue to work.

Increased an op count in test_mobile_optimizer.py because now we need aten::format to form the actual exception message.

This is built upon an WIP PR:  https://github.com/pytorch/pytorch/pull/34112 by driazati

Pull Request resolved: https://github.com/pytorch/pytorch/pull/41907

Reviewed By: ngimel

Differential Revision: D22778301

Pulled By: gmagogsfm

fbshipit-source-id: 2b94f0db4ae9fe70c4cd03f4048e519ea96323ad

torch/_lobpcg.py

@@ -377,9 +377,8 @@ class LOBPCG(object):
                 # strict ordering of eigenpairs
                 break
             count += 1
-        assert count >= prev_count, (
-            'the number of converged eigenpairs '
-            '(was %s, got %s) cannot decrease' % (prev_count, count))
+        assert count >= prev_count, 'the number of converged eigenpairs ' \
+            '(was {}, got {}) cannot decrease'.format(prev_count, count)
         self.ivars['converged_count'] = count
         self.tvars['rerr'] = rerr
         return count
@@ -723,10 +722,14 @@ class LOBPCG(object):
             if rerr < tau_ortho:
                 break
             if m < U.shape[-1] + V.shape[-1]:
+                # TorchScript needs the class var to be assigned to a local to
+                # do optional type refinement
+                B = self.B
+                assert B is not None
                 raise ValueError(
                     'Overdetermined shape of U:'
                     ' #B-cols(={}) >= #U-cols(={}) + #V-cols(={}) must hold'
-                    .format(self.B.shape[-1], U.shape[-1], V.shape[-1]))
+                    .format(B.shape[-1], U.shape[-1], V.shape[-1]))
         self.ivars['ortho_i'] = i
         self.ivars['ortho_j'] = j
         return U