fix(vlm): correct CP gradients and vision sharding (#3414)

* fix(vlm): correct CP gradients and vision sharding

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* fix(vlm): simplify CP loss and gradient handling

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* test: align loss fixtures with model contracts

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* docs: document vision frame sharding thresholds

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* docs: apply tech pubs copyedits

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

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Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>
Co-authored-by: Alexandros Koumparoulis <153118171+akoumpa@users.noreply.github.com>