Neg partial log likelihood loss is 0 every time the batch size is 1

[mayurmallya]

Hi there,

Thanks for sharing this wonderful library!

I was trying to run a survival analysis using the Cox proportional hazards model and due to the GPU constraints, I have to go with the batch size of 1. And every time I run the model, I observe that the loss value is always 0 when I'm using cox.neg_partial_log_likelihood.

I looked into the implementation of the _partial_likelihood_cox and it seems that the log_denominator gets the same value as the log_hz_sorted when the batch size is 1, resulting in the loss to be 0.

I was wondering if there is a workaround for this issue, please let me know. Also attaching the link to the corresponding code https://github.com/Novartis/torchsurv/blob/799eb30f4b123f4871659ffcfbb7c914d1f67fcd/src/torchsurv/loss/cox.py#L174

Thank you in advance!