Add custom_loss_func and custom_evaluate_func to trainer with example

[yinpu]

#50 Implemented a custom CVR loss function to accurately reflect the conditional nature of conversion rate calculations. The loss is computed only for instances where click=1, aligning with the typical behavior expected in conversion rate optimization scenarios. This update includes the core implementation of the custom loss function along with examples demonstrating its application and efficacy in relevant contexts.

[1985312383]

你好 @yinpu ，很抱歉这么久之后才梳理这个PR
我想问一下这里的设计，是为了避免排序模型中有些优化目标是CVR，而不是CTR时，设计的可自定义损失函数的接口么？
我想问，是否在复现模型时，将损失函数定义在模型里，是否更加方便该模型的复现，同时避免了当使用不同的模型时，我们需要单独的在训练器这里去写损失函数，对项目不熟悉的同学可能难度太大。如果我们提前写好各个模型的损失函数在训练器这里，会使项目看起来很凌乱？