Fix causal convolution padding for even dilated kernels

[YaoxinHuang]

CausalConv1d(kernel_size=4, dilation=2) pads seven frames although its receptive field needs six, so both left and right modes produce an extra frame and fail the length assertion. This also prevents a causal HiFT ResBlock from using an even kernel with an even dilation.

Use (kernel_size - 1) * dilation for the padding. CPU regressions cover left/right output alignment, explicit context, causal direction, existing odd-kernel/dilation-one cases, and a real ResBlock forward/backward pass.

Validation: python -m unittest discover -s tests -p test_causal_convolution.py -v — four methods pass on Python 3.10 / PyTorch 2.3.1 CPU; the original code reports 11 failures including subtests. Changed files pass the repository's flake8 settings. Full-repository flake8 reports the same 56 existing findings before and after the change; the workflow whitespace scans likewise match the baseline.

Prepared and tested with Codex.