What's the difference between torch.stack() and torch.cat() functions?

OpenAI's REINFORCE and actor-critic example for reinforcement learning has the following code:

REINFORCE:

policy_loss = torch.cat(policy_loss).sum()

actor-critic:

loss = torch.stack(policy_losses).sum() + torch.stack(value_losses).sum()

One is using torch.cat, the other uses torch.stack.

As far as my understanding goes, the doc doesn't give any clear distinction between them.

I would be happy to know the differences between the functions.

#python #machine-learning