Fix spatial_ndim after EnsureChannelFirst adds a channel dim (fixes #…

[sillyfellow21]

Fixes #9184

Description

EnsureChannelFirst(channel_dim="no_channel") adds a channel dim to a (H, W, D) tensor, but the cached spatial_ndim stays at ndim - 1 = 2 because the constructor assumes dim 0 is already the channel. Any later spatial crop then shifts only the first two axes and silently drops the third axis' affine offset — e.g. a crop origin lands at [15, 25, 30] instead of [15, 25, 35]. The same bug affects 2D input (spatial_ndim was 1 instead of 2).

This patch recomputes spatial_ndim from the affine the moment EnsureChannelFirst adds the channel dim, so downstream transforms (e.g. SpatialCrop) build the full N-axis translation. It only touches the no-channel case and non-batched affines, so nothing else is affected.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

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