Cleanup conversion transforms

[pmeier]

This PR stems from the discussion in #6783 (comment). @datumbox and I agreed to the following things:

• Let's not have a copy flag on our convert_* kernels. It is currently unused for convert_color_space. It is used extensively for convert_bounding_box in our kernels due the idiom of converting to a specific format, performing the computation, and converting back. In the future, we wanted to have a look at these kernels anyway for possible performance gains by implementing the computation for every format. After this PR the current implementation like

  bounding_box = convert_format_bounding_box(
      bounding_box, old_format=format, new_format=features.BoundingBoxFormat.XYXY
  )

  changes to

  bounding_box = (
      bounding_box.clone()
      if format == features.BoundingBoxFormat.XYXY
      else convert_format_bounding_box(bounding_box, old_format=format, new_format=features.BoundingBoxFormat.XYXY)
  )

• Let's not have any .to_*() methods on the features. While useful for debugging, they go against the other idioms we are using. We can always re-add them later if there is user demand for it. Note that Label.to_categories() is an exception since it has no transformation associated with it.

cc @vfdev-5 @datumbox @bjuncek

[vfdev-5]

bounding_box = (
    bounding_box.clone()
    if format == features.BoundingBoxFormat.XYXY
    else convert_format_bounding_box(bounding_box, old_format=format, new_format=features.BoundingBoxFormat.XYXY)
)

IMO, this is too bulky. I do not mind to have copy in convert_format_bounding_box and reduce lines of user's code...

[datumbox]

Thanks @pmeier, looks good overall. Just a couple of minor comments.

[pmeier]

@vfdev-5

I do not mind to have copy in convert_format_bounding_box and reduce lines of user's code...

Same here with the focus on consistency. If one convert_* kernel or dispatcher has a copy flag, every should have it. Currently we have copy: bool = True, which has a runtime implication unless someone explicitly sets the copy=False in case no copy is needed. Meaning convert_dtype of #6783 (comment) will be slower by default than before for mostly no reason. This PR basically hard-codes copy=False.

Vasilis argument was that besides for convert_format_bounding_box we aren't using the copy flag anywhere. Even that is only used a handful of times throughout our API. Thus, if we want consistency, it is fairly simple to remove and avoids dead code. I don't want to misrepresent the argument, so I let him chime in here. cc @datumbox

I'm ok with or without the copy flag if we are consistent with it. I'll let the two of you hash it out.

[datumbox]

LGTM, only one comment below.

[vfdev-5]

@datumbox @pmeier can we do the following:

• make all conversion ops like _xywh_to_xyxy inplace (without cloning internally) and
• we can apply .clone() for the output of convert_format_bounding_box

bounding_box = convert_format_bounding_box(
    bounding_box.clone(), old_format=format, new_format=features.BoundingBoxFormat.XYXY
)

?

[datumbox]

@vfdev-5 that would make our only operator that does in-place ops (outside of the optimizations that have no effect). If we were to go down that route, purely for speed reasons, I would recommend adopting the inplace argument for this one kernel (because it's used so often in conversions). The default should be inplace=False in that case. I would do this outside of this PR though after measuring its speed.