sum_dim throws exception

Hi,

I have an exception with is throwed by the following code:
`auto agg_v2 = dynet::sum_dim(v2, {1});`

The exception is:
```
terminate called after throwing an instance of 'std::invalid_argument'
what(): Out of bounds exception in Dim::delete_dims
```
The dimension of v2 is {200,17}. From the stack trace it is thrown by this line on of code: https://github.com/clab/dynet/blob/master/dynet/dim.h#L238

I am running on GPU with autobatch. I don't really know what else I can provide unfortunately...

Start with dynet/dim.h at line 238 and trace the dimensions produced by dynet::sum_dim(v2, {1}) for v2 shaped {200,17}. Reproduce the failure using GPU with autobatch enabled, then use the reported stack trace and issue discussion to isolate the cause; done means the call no longer throws unexpectedly and the behavior is covered by a regression test.

Written by the indexing model from the issue text.