Change custom_skip_targets meaning for constant_prop_pass (#3491)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3491

Some users of `constant_prop_pass` want to fold across calls to
`full`, because representing a tensor as a program constant is a requirement
for some backends.
This came up when writing some tests using `torch.ones` as a weight tensor,
which is represented as `aten.full` in Edge Dialect.

When the user specifies a custom skip set, do *not* add the default `aten.full`
function, in case the user doesn't want it.

Reviewed By: angelayi

Differential Revision: D56894215

fbshipit-source-id: 24b7f570ce41576650c457586fc5540371889121