For binary labels y_true/y_pred (0s and 1s), compute precision, recall,
and F1 score by hand from true/false positives/negatives. Return a length-3 tensor
[precision, recall, f1].
Input
y_true = tensor([1, 0, 1, 1, 0, 1])
y_pred = tensor([1, 0, 0, 1, 0, 0])
Output
tensor([1.0000, 0.5000, 0.6667])