How to add a new dimension to a PyTorch tensor?

a = torch.zeros(4, 5, 6)
a = a[:, :, None, :]
assert a.shape == (4, 5, 1, 6)

You can add a new axis with torch.unsqueeze() (first argument being the index of the new axis):

>>> a = torch.zeros(4, 5, 6)
>>> a = a.unsqueeze(2)

>>> a.shape
torch.Size([4, 5, 1, 6])

Or using the in-place version: torch.unsqueeze_():

>>> a = torch.zeros(4, 5, 6)
>>> a.unsqueeze_(2)

>>> a.shape
torch.Size([4, 5, 1, 6])

x = torch.tensor([1, 2, 3, 4])
y = torch.unsqueeze(x, 0)

y will be -> tensor([[ 1, 2, 3, 4]])

EDIT: see more details here: https://pytorch.org/docs/stable/generated/torch.unsqueeze.html