Remove 1024 limit for str on collective ops

[vfdev-5]

🚀 Feature

Currently, if we have a limitation of 1024 chars for str input on collective ops:

ignite/ignite/distributed/comp_models/base.py

Lines 92 to 95 in 314b87f

	size = 1024
	if len(x) > size:
	warnings.warn(f"Input string size {len(x)} is larger than {size} and thus will be truncated")
	x = x[:size]

The idea is to remove this limitation. We can do that as following:

• for all inputs as str, we can first gather all lenghts
• find the max lengths
• pad the input to the max lenghts + add input length value (as it is done currently)
• perform collective op

Bug ref: Project-MONAI/MONAI#1633

[sdesrozis]

ignite/ignite/distributed/comp_models/base.py

Line 185 in 314b87f

tensor = torch.empty(1, 1025, device=self.device(), dtype=torch.long)

Maybe a fix has to be done here.