Pt2.6 compatibility

# Before submitting - [x ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements) - [ x] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/main/CONTRIBUTING.md)? - [ ] Did you make sure to update the docs? (no need) - [ ] Did you write any new necessary tests? (no need) ## What does this PR do? Fixes #5610. The default value for the weights_only argument in torch.load() changed from False to True between PyTorch versions 2.5 and 2.6. By explicitly including weights_only=False in the torch.load calls, the code is kept working in the same way as previously also in PyTorch >= 2.6.