redefine 1D `self.N`, deprecate`self.J_pad`?

in 1D, self.N is the unpadded length of the input signal. It is defined in ScatteringBase1D as some derivative of self.shape and isn’t being used anywhere else.

Same for 2D where self.shape unpacks as self.M, self.N.

I see no reason to keep these values as attributes in the object when:

• we have self.shape for that
• these values are only being used transiently, in self.build()
• our filterbank construction relies on N_padded, not N
• these values are trivially available in the forward via self.N = len(x)

In the spirit of preparing for scattering on variable-length inputs (shape=None, a long-requested feature !), i ask the permission to remove these attributes from self.

I am open to making a deprecation cycle where we convert these attributes to read-only properties and print a DeprecationWarning in v0.3 if anyone tries to access these numbers (NB: none of our examples access self.N and friends)

@janden @eickenberg

[janden]

Might be good to rename it to something like _N_padded, though to make clear it’s the padded size that’s intended.

[janden]

But why even keep the N attribute? The padded size is, in a sense, an internal parameter for the implementation. Why do we need to expose it to the user at all?