[Bug] TypeError when calling `backward` on `gpytorch.functions.logdet` 
# 🐛 Bug

I think there's a missing `list()` in https://github.com/cornellius-gp/gpytorch/blob/master/gpytorch/functions/_inv_quad_log_det.py#L221. I'm not super familiar with gpytorch internals so hopefully this is correct -- if so, happy to contribute the one-liner fix. 

## To reproduce

** Code snippet to reproduce **
```python
### works (I'm guessing something dispatches elsewhere for small matrices?)
import torch
from torch.autograd import backward
import gpytorch
from gpytorch.functions import logdet, inv_matmul
n = 100
inp = torch.arange(n, dtype=torch.float)
kern = gpytorch.kernels.RBFKernel()(inp)
ld = logdet(kern)
backward(ld)

### doesn't work
import torch
from torch.autograd import backward
import gpytorch
from gpytorch.functions import logdet, inv_matmul
n = 1000
inp = torch.arange(n, dtype=torch.float)
kern = gpytorch.kernels.RBFKernel()(inp)
ld = logdet(kern)
backward(ld)
```

** Stack trace/error message **
```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-46-593fbced29ac> in <module>()
      3 kern = gpytorch.kernels.RBFKernel()(inp)
      4 ld = logdet(kern)
----> 5 backward(ld)

<PATH SNIPPED>/lib/python3.7/site-packages/torch/autograd/__init__.py in backward(tensors, grad_tensors, retain_graph, create_graph, grad_variables)
     91     Variable._execution_engine.run_backward(
     92         tensors, grad_tensors, retain_graph, create_graph,
---> 93         allow_unreachable=True)  # allow_unreachable flag
     94 
     95 

<PATH SNIPPED>/lib/python3.7/site-packages/torch/autograd/function.py in apply(self, *args)
     75 
     76     def apply(self, *args):
---> 77         return self._forward_cls.backward(self, *args)
     78 
     79 

<PATH SNIPPED>lib/python3.7/site-packages/gpytorch/functions/_inv_quad_log_det.py in backward(ctx, inv_quad_grad_output, logdet_grad_output)
    221             res = matrix_arg_grads
    222 
--> 223         return tuple([None] * 9 + res)

TypeError: can only concatenate list (not "tuple") to list
```

## Expected Behavior

No error. 

## System information

**Please complete the following information:**
- GPyTorch version: 0.3.2 <!-- GPyTorch Version (run `print(gpytorch.__version__)` -->
- pytorch version 1.1.0. 
- Mac OSX. 


diff --git a/gpytorch/functions/_inv_quad_log_det.py b/gpytorch/functions/_inv_quad_log_det.py
index 850986e1..0629f5b9 100644
--- a/gpytorch/functions/_inv_quad_log_det.py
+++ b/gpytorch/functions/_inv_quad_log_det.py
@@ -218,6 +218,6 @@ class InvQuadLogDet(Function):
         if ctx.inv_quad:
             res = [inv_quad_rhs_grad] + list(matrix_arg_grads)
         else:
-            res = matrix_arg_grads
+            res = list(matrix_arg_grads)
 
         return tuple([None] * 9 + res)
['gpytorch/functions/_inv_quad_log_det.py']