HEALPix UNet code cleanup

[yikwill]

Several HEALPix settings were duplicated across YAML configs, mutated at build time, or lived on monolithic dataclasses where variant-specific fields were silently ignored. This PR makes configs easier to load from YAML, reason about, and extend. It refactors HEALPix UNet configuration and build path to address feedback from a previous HEALPix PR. Block configs are split into dacite discriminated unions and runtime/build-time concerns are separated.

Changes:
Build-time vs config-time separation

• Channel counts (input_channels / output_channels) have been removed from UNetEncoderConfig, UNetDecoderConfig, and conv/downsample/upsample block configs. Insteady, they are now passed explicitly to .build() from HEALPixUNet / HEALPixUNetBuilder.
• Config objects are no longer mutated (e.g. via .replace) when constructing the model.

Shared build context

• Added HEALPixBuildContext and HEALPixLayerBuildContext for model-level settings (hpx_padding_mode, nside per level). They are configured once on HEALPixUNet / HEALPixUNetBuilder, then the encoder, decoder, and blocks receive per-level views via ctx.layer(level).

Discriminated union block configs

• Monolithic block configs split into one dataclass per block_type, each with a Literal[...] discriminator for dacite union resolution. For example, the union alias DownSamplingBlockConfig has variants MaxPoolDownsamplingBlockConfig, AvgPoolDownsamplingBlockConfig, and DealiasedDownsampleBlockConfig. Each variant exposes only fields relevant to that block type.

Other

• enable_nhwc option removed. Memory format of inputs is already hard coded.

• Replace th calls with torch for clarity.

• Tests added

• If dependencies changed, "deps only" image rebuilt and "latest_deps_only_image.txt" file updated