Input dims may not be known in advance

See !93 (merged)

I have the following proposition to overcome the current too static graph handling behavior in Aidge:

• Allow operators to acknowledge output dims forwarded even if the dimension if not known in advance, provided it is known that the output dims will be known during execution;
• Adapt the producer-consumer model to also handle an abstract "token" unit that can be mixed with data size, in order to allow static scheduling of nodes with unknown quantity of data production/consumption before execution. So, operators with undefined input/output dims will consume/produce a token for these inputs/outputs.