Adds nin and nout properties to unary and binary element-wise classes

[ndgrigorian]

The nin and nout properties return the number of arguments to the element-wise function treated as inputs or outputs, respectively.

nout is always 1 for both unary and binary functions, as both cases permit an out keyword argument.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1478/index.html

[coveralls]

coverage: 85.962% (+0.02%) from 85.942%
when pulling ad258b8 on nin-nout-properties
into 5ec9fd5 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.15.1dev2=py310h15de555_19 ran successfully.
Passed: 875
Failed: 56
Skipped: 59