MultiTask models are not handling evaluation metrics correctly

[britojr]

Describe the bug(问题描述)

When dealing with MultiTask, i.e. when there is more than one target variable, the evaluation metrics are not being correctly applied. As we can see on this line the evaluation metric is applied as if y was a single column.

To Reproduce(复现步骤)
Steps to reproduce the behavior:

1. Copy the data and example code on the documentation page for MultiTask model MMOE
2. Execute the code as is and see the warning: UserWarning: The y_pred values do not sum to one. Make sure to pass probabilities.
3. The warning indicates that that the columns in y_pred are being interpreted as a single probability distribution instead of two distinct distributions for each of the binary tasks.

Operating environment(运行环境):

• python version: 3.12.4
• torch version: 2.5.1
• deepctr-torch version: 0.2.9