Handle transposed variables properly in atoms

Right now, transposed variables in atoms lead to the atom being designated as not being proxable. A good example of this is doing something like 

```python
from rlaopt.atoms import NucNorm
from rlaopt.expression import Variable

x = Variable(torch.randn(2, 3))
nuc_norm = NucNorm(x.T)
nuc_norm.is_proxable()
```

This should be fixed -- transposing a variable should not affect proxability!