Propagate zeros from Conv layers to multiplication config

[bo3z]

Description

:memo: Convolutional layers use Dense matrix multiplication at its core, but do not fully utilise the benefit of n_zeros in the Latency strategy of Dense layers.

• When there are zeros, HLS can optimise the number of DSPs in Latency strategy, by using the resource pragma.
• This is currently done for Dense layers and works quite well, even for RF != 1 (Tested as part of #768 )
• However, n_zeros is always set to zero for Conv2D layers.

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)

Tests

• No new tests, the current PyTests should verify no changes in Conv implementations
• I can add some results from my synthesis of #768 to verify changes are correct.

Checklist

• [x] I have read the guidelines for contributing.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] I have installed and run pre-commit on the files I edited or added.
• [x] I have added tests that prove my fix is effective or that my feature works.