The answer is that both phrases use IDIOM as the principal literary technique.
An idiom is a figure of speech that consists of a phrase that means something other than its literal meaning.
In the first sentence, Byron refers to a person who truly loves by means of the expression: "a heart whose love is innocent!". In the second sentence, Byron speaks of a woman being beautiful through the expression "she walks in beauty?"
However, we find other literary techniques:
A heart whose love is innocent
- The first rhetorical figure is a metonymy. / a heart /
Metonymy is a rhetorical figure of thought that consists of designating one thing with the name of another with which there is a relation of spatial contiguity. In this case, the part / the heart / for the whole / the person / is named /
- The second rhetorical figure is a personification/love is innocent /
The personification is a rhetorical figure that consists of attributing human qualities to substantive non-human ones. In this case, the abstract noun love is attributed to the human quality of innocence.
She walks in beauty
- The rhetorical figure of this fragment is a metaphor. A metaphor is a figure of speech, which compares two things without the use of 'like' or 'as'.