# Buggy code
def precondition_closure(tensor):
    qqt = self._q_cache.matmul(self._q_cache.transpose(-2, -1).matmul(tensor))
    if self._constant_diag:
        return (1 / self._noise) * (tensor - qqt)
    return (tensor / self._noise) - qqt


# Traceback (RuntimeError)
RuntimeError                              Traceback (most recent call last)
<ipython-input-4-9e151e2de37a> in <module>
24 with gpytorch.settings.max_cholesky_size(100), gpytorch.settings.min_preconditioning_size(100):
25     train_dist = gp(train_x)
---> 26     loss = -mll(train_dist, train_y).sum()

~/Code/gpytorch/gpytorch/module.py in __call__(self, *inputs, **kwargs)
26
27     def __call__(self, *inputs, **kwargs):
---> 28         outputs = self.forward(*inputs, **kwargs)
29         if isinstance(outputs, list):
30             return [_validate_module_outputs(output) for output in outputs]

~/Code/gpytorch/gpytorch/mlls/exact_marginal_log_likelihood.py in forward(self, function_dist, target, *params)
49         # Get the log prob of the marginal distribution
50         output = self.likelihood(function_dist, *params)
---> 51         res = output.log_prob(target)
52
53         # Add additional terms (SGPR / learned inducing points, heteroskedastic likelihood models)

~/Code/gpytorch/gpytorch/distributions/multivariate_normal.py in log_prob(self, value)
138
139         # Get log determininat and first part of quadratic form
--> 140         inv_quad, logdet = covar.inv_quad_logdet(inv_quad_rhs=diff.unsqueeze(-1), logdet=True)
141
142         res = -0.5 * sum([inv_quad, logdet, diff.size(-1) * math.log(2 * math.pi)])

~/Code/gpytorch/gpytorch/lazy/lazy_tensor.py in inv_quad_logdet(self, inv_quad_rhs, logdet, reduce_inv_quad)
1069             probe_vectors,
1070             probe_vector_norms,
-> 1071             *args,
1072         )
1073

~/Code/gpytorch/gpytorch/functions/_inv_quad_log_det.py in forward(ctx, representation_tree, dtype, device, matrix_shape, batch_shape, inv_quad, logdet, probe_vectors, probe_vector_norms, *args)
65         lazy_tsr = ctx.representation_tree(*matrix_args)
66         with torch.no_grad():
---> 67             preconditioner, precond_lt, logdet_correction = lazy_tsr._preconditioner()
68
69         ctx.preconditioner = preconditioner

~/Code/gpytorch/gpytorch/lazy/added_diag_lazy_tensor.py in _preconditioner(self)
84                 )
85                 return None, None, None
---> 86             self._init_cache()
87
88         # NOTE: We cannot memoize this precondition closure as it causes a memory leak

~/Code/gpytorch/gpytorch/lazy/added_diag_lazy_tensor.py in _init_cache(self)
107             self._init_cache_for_constant_diag(eye, batch_shape, n, k)
108         else:
--> 109             self._init_cache_for_non_constant_diag(eye, batch_shape, n)
110
111         self._precond_lt = PsdSumLazyTensor(RootLazyTensor(self._piv_chol_self), self._diag_tensor)

~/Code/gpytorch/gpytorch/lazy/added_diag_lazy_tensor.py in _init_cache_for_non_constant_diag(self, eye, batch_shape, n)
125         # With non-constant diagonals, we cant factor out the noise as easily
126         # eye = eye.expand(*batch_shape, -1, -1)
--> 127         self._q_cache, self._r_cache = torch.qr(torch.cat((self._piv_chol_self / self._noise.sqrt(), eye)))
128         self._q_cache = self._q_cache[..., :n, :] / self._noise.sqrt()
129

RuntimeError: Tensors must have same number of dimensions: got 3 and 2

# Fixed code
def precondition_closure(tensor):
    # This makes it fast to compute solves with it
    qqt = self._q_cache.matmul(self._q_cache.transpose(-2, -1).matmul(tensor))
    if self._constant_diag:
        return (1 / self._noise) * (tensor - qqt)
    return (tensor / self._noise) - qqt