Unify metrics output type

🚀 Feature

The idea is to verify the output type for all metrics (output of compute function) and update the docs accordingly.

In general, metric's output should be a float number. In some particular cases, like Recall/Precision with average=False, the output is a torch tensor. So, let's see and decide if the output of compute() method can be :

• def compute() -> float
• def compute() -> Union[float, torch.Tensor] and tensor is on CPU
• def compute() -> torch.Tensor with tensor on CPU

To address this FR, we have to make sure for each metric what kind of type it supposes to return and update the docs accordingly.