Fix weight precision format string

[vloncar]

Description

Observed in #874 but not limited to zero-integer quantizers, the generated HLS doesn't achieve bit-perfect matching with the QKeras model. This turns out to be due to the initialization of the weights from literals. We print literals with {:#f} (where # is the number of digits required), but this doesn't work correctly and we should instead use {:.#f} (with the leading .). For example:

Python 3.8.5 (default, Sep  4 2020, 07:30:14) 
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x = 0.3203125
>>> '{:7f}'.format(x)
'0.320312'
>>> '{:.7f}'.format(x)
'0.3203125'
>>>

ap_fixed<8,1> would interpret the literal 0.320312 as a value 0.3125, messing up the result. This one-liner (in fact one-character) fixes #874 (and alike).

Type of change

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

Tests

Existing QKeras tests should suffice, we should eventually aim for a stricter agreement, ideally bit-perfect one, but that's a separate change.

Checklist

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

[lgray]

Just tried it for my model with quantized_bits(N, 0, alpha=1) everywhere and it looks good. Thanks!

[lgray]

Though for some reason I need wider widths than expected to achieve closure, as described in slack chat.