Deprecate fancy decorator syntax in batch transforms

[eddddddy]

Context:
Currently we allow the user to do something like

@qml.batch_input(argnum=0)
@qml.qnode(dev)
def circuit(inputs):
    ...

The only reason this works is because of some custom magic in qml.batch_transform that wraps the arguments in a partial call and returns that wrapper.

With the new transform program we want to switch to using the following syntax:

@partial(qml.batch_input, argnum=0)
@qml.qnode(dev)
def circuit(inputs):
    ...

which works identically but is easier to understand directly from the documentation of qml.batch_input.

Description of the Change:

• Add deprecation warning for the use of the first syntax
• Allow the first syntax in the new transform API so that existing batch transforms can be ported over without breaking things

Benefits:
Less confusing

Possible Drawbacks:
Slightly more typing for users

[rmoyard]

💯 Thanks for that. Just a couple of comments

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (39e6577) 99.65% compared to head (d4548af) 99.65%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4457   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files         376      376           
  Lines       33135    33142    +7     
=======================================
+ Hits        33022    33029    +7     
  Misses        113      113           

Files Changed	Coverage Δ
pennylane/transforms/core/transform_dispatcher.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[albi3ro]

@trbromley @rmoyard What do we think of always advertising using keyword arguments instead of using positional arguments?

@partial(qml.transforms.batch_params, all_operations=True)
@qml.qnode()
...

It removes all of this ambiguity around positional arguments.

[rmoyard]

No problems with advertising kwargs 👍

[timmysilv]

can you add a test or two that uses the @partial(transform_fn, **kwargs) syntax? i'm just worried that once we drop the warning, we'll delete those tests and lose some coverage. otherwise I think the errors are very clear, and it all looks great!