note on `diff`

[keyboardAnt]

Thanks for the open source contribution!

One small note is that the diff_spec function differs from common library standards.

• it returns N elements, while numpy/pytorch return N-1.
• it seeds out[0] with the first value, while pandas uses NaN.

implementation	length	first element
diff_spec	N	a[0]
numpy / pytorch	N-1	a[1] - a[0]
pandas	N	NaN