[Relay] Add logical operators

[abergeron]

This is a follow-up to #2453, to port the operators that were added to Relay.

The one thing I'm less than certain about is the support_level since I've yet to see some actual documentation about what it means.

[slyubomirsky]

I would venture to guess (though I am not an authority on it) that a support level of 1 would be appropriate for core, basic operators like the logical connectives

Per Tianqi's point below, consistency with existing logical ops is better

[tqchen]

See https://docs.tvm.ai/langref/relay_op.html for support level explaination. I think it should be level 4

[srkreddy1238]

@abergeron look good to me.

Update the support_level as per @tqchen advice. Also update the support_level for NNVM too.

[MarisaKirisame]

Should it be defined using RELAY_REGISTER_BINARY_OP? It mean it work on any dtype.

[abergeron]

I've fixed the support level.

I don't think those operators are applicable for all types, but I don't know how to limit the types.

[tqchen]

Thanks, @MarisaKirisame @abergeron @srkreddy1238 @slyubomirsky , this is now merged.

We could use a followup PR to update type relation to assert bool types, but since it is not strictly necessary, we can go with the current ver for now

[tqchen]

@abergeron please send a followup to update https://docs.tvm.ai/langref/relay_op.html

[MarisaKirisame]

@tqchen I can do it, but I have tons of task next week. I will open a issue. If noone does it next week I will pick it up, does it sound good?

[tqchen]

@MarisaKirisame Sounds good, go ahead and open an issue