Migration note : v0.3.0 to v0.4.0

Migration from v0.3.0 to v0.4.0 note

PyTorch-Ignite v0.4.0 has several backward compatibility breaking changes.

Simplified Engine (https://github.com/pytorch/ignite/pull/940)

Engine in v0.3.0 provided automatically a dataflow synchronization during the run to manage a reproducible training (more info here). However, this approach has several important drawbacks that we decided to remove this behaviour in Engine of v0.4.0.