Add TF glu activation function
# What does this PR do?

Bite-sized PR that adds the GLU activation function for TF, as defined in the original paper. It is a requirement for a few speech models, like `Speech2Text`.

We can see that it matches PyTorch's implementation
```python
import numpy as np
import torch
from transformers.activations_tf import glu
a = np.asarray([[1., 2., 3., 4.], [5., 6., 7., 8.]])
assert np.allclose(torch.nn.functional.glu(torch.tensor(a), dim=0).numpy(), glu(a, axis=0).numpy())
assert np.allclose(torch.nn.functional.glu(torch.tensor(a), dim=1).numpy(), glu(a, axis=1).numpy())
assert np.allclose(torch.nn.functional.glu(torch.tensor(a)).numpy(), glu(a).numpy())
```

After I get one model using it fully operational, I'm going to attempt to add it to Keras 😎 
A little late but looks good to me!