[Bug] Cannot serialize/deserialize SmoothedBoxPrior when some args are broadcast
# 🐛 Bug

It seems like `SmoothedBoxPrior` for >1d doesn't work with serialization/deserialization when only some args are broadcast. 

## To reproduce

```python
import torch
import gpytorch
pr = gpytorch.priors.SmoothedBoxPrior(torch.zeros(2), torch.ones(2))
pr.load_state_dict(pr.state_dict())
```

** Stack trace/error message **
```
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-5-6b4b2e881beb> in <module>
      2 import gpytorch
      3 pr = gpytorch.priors.SmoothedBoxPrior(torch.zeros(2), torch.ones(2))
----> 4 pr.load_state_dict(pr.state_dict())

<...PATH..>/torch/nn/modules/module.py in load_state_dict(self, state_dict, strict)
    877         if len(error_msgs) > 0:
    878             raise RuntimeError('Error(s) in loading state_dict for {}:\\n\\t{}'.format(
--> 879                                self.__class__.__name__, "\\n\\t".join(error_msgs)))
    880         return _IncompatibleKeys(missing_keys, unexpected_keys)
    881 

RuntimeError: Error(s) in loading state_dict for SmoothedBoxPrior:
	While copying the parameter named "sigma", whose dimensions in the model are torch.Size([2]) and whose dimensions in the checkpoint are torch.Size([2]), an exception occured : ('unsupported operation: more than one element of the written-to tensor refers to a single memory location. Please clone() the tensor before performing the operation.',).

```

Note that `SmoothedBoxPrior(a=torch.zeros(2), b=torch.ones(2), sigma=torch.ones(2)*0.01)` succeeds, as does `gpytorch.priors.GammaPrior(torch.ones(2),1)`.

## Expected Behavior

Successful load. 

## System information

**Please complete the following information:**
- gpytorch version: 1.1.1
- pytorch version: 1.5.0
- OS: tested on Centos and Mac OSX. 



diff --git a/gpytorch/priors/smoothed_box_prior.py b/gpytorch/priors/smoothed_box_prior.py
index 3a2d8292..58ad3327 100644
--- a/gpytorch/priors/smoothed_box_prior.py
+++ b/gpytorch/priors/smoothed_box_prior.py
@@ -47,7 +47,7 @@ class SmoothedBoxPrior(Prior):
         del self.a, self.b, self.sigma
         self.register_buffer("a", _a)
         self.register_buffer("b", _b)
-        self.register_buffer("sigma", _sigma)
+        self.register_buffer("sigma", _sigma.clone())
         self.tails = NormalPrior(torch.zeros_like(_a), _sigma, validate_args=validate_args)
         self._transform = transform
 
['gpytorch/priors/smoothed_box_prior.py']