list index out of range in _sync_dist

[geoffrey-g-delhomme]

🐛 Bug

list index out of range out of range error when using a list state with a default=[].

To Reproduce

self.add_state("p_true_positives", default=[], dist_reduce_fx="cat")

when no value is added during update, it raises a list index out of range when hitting:

if isinstance(output_dict[attr][0], Tensor): in metric.py (l. 374).

Code sample

See above

Expected behavior

Just add a len() > 0 check before

Environment

• TorchMetrics version (and how you installed TM, e.g. conda, pip, build from source): 0.9.3, installed with pip
• Python & PyTorch Version (e.g., 1.0): torch 1.12.1 / python 3.10.4
• Any other relevant information such as OS (e.g., Linux): Linux

Additional context

[github-actions]

Hi! thanks for your contribution!, great first issue!

[stancld]

Hi @geoffrey-g-delhomme, can you share some minimal reproducible code please :]