Better typing for Engine

[vfdev-5]

🚀 Feature

Improve typing of Engine module, especially for event_name.
Currently, it is specified as str, but actually can be anything if registered:

from ignite.engine.events import Events
from ignite.engine import Engine

e = Engine(lambda e, b: None)

class A: pass
a = A()

e.register_events("up", "down")
e.register_events(1, 2, 3)
e.register_events(a)

print(e._allowed_events)
e.add_event_handler("up", lambda e: print("up"))
e.add_event_handler(1, lambda e: print("1"))
e.add_event_handler(a, lambda e: print("a"))

e.fire_event("up")
e.fire_event(1)
e.fire_event(a)

Let's discuss here if we would like to restrict event_type to some subset of types or leave as any type.

@sdesrozis, @ykumards
cc @kai-tub as you said you were big fan of typing

[sdesrozis]

First step, type Any should be better than nothing.
How does this relate to CustomEvent ?

[sdesrozis]

I suggest to use Any rather than str (and used with int, etc.)

@vfdev-5 @kai-tub @ykumards ??

[vfdev-5]

@sdesrozis go ahead with a PR, please