Minor import typo

Just change the line

```python
from torchrl.modules import MLP
```

to

```python
from torchrl.modules import MLP, LSTMModule
```

So the example doesn't raise an exception.

https://github.com/pytorch/rl/blame/0592fdc56f400ba6feedbf7b3b77b992ac6b06e1/torchrl/modules/tensordict_module/rnn.py#L396