* Why?

Persistent multimodal encoder cache is currently all-or-nothing per
`MultimodalParams`: if any item of a request misses the cache, every
item is re-encoded. In practice this makes the cache inert for common
VLM workloads where consecutive requests share most of their images
but differ in one -- e.g. multi-turn conversations that grow context,
or comparison tasks that anchor on a shared reference set.

* What?

Add an opt-in per-item partial-hit path to `MultimodalModelMixin`:

- `EncoderCachePartition` records the per-item cache lookup result
  (hits / miss indices / keys).
- `_partition_encoder_cache` replaces `_attach_encoder_cache_hit` and
  records every item lookup rather than bailing on first miss.
- `_encode_with_partial_cache` encodes only the miss items via a
  residual `MultimodalParams`, then interleaves cached and freshly
  encoded rows in original per-item order so downstream
  `get_multimodal_embeddings` treats the param as fully cached.
- `_apply_metadata_slice` re-slices `multimodal_embedding_lengths`
  and `multimodal_hashes` on the residual so every model gets the
  parallel per-item metadata slice identically.
- Models opt in with `supports_granular_encoder_cache = True` and
  implement `slice_multimodal_data_items`, which slices raw modality
  tensors (`pixel_values`, `image_grid_thw`, ...) for the given
  item indices. Default remains `False`; existing models continue on
  the all-or-nothing path unchanged.

Onboard Mistral 3 (Mistral-Small-3.1-24B) as the first opted-in model
by slicing `pixel_values` on dim 0 and `image_sizes` list in parallel.

* Numbers

Real trtllm-serve run against `mistralai/Mistral-Small-3.1-24B-Instruct-2503`
on 1x H200 (BF16, TP=1, max_seq_len=65536, encoder_cache_max_bytes=2GiB).
Two requests each with 100 items of 512x512 images; request 2 shares
99 items with request 1 and adds 1 novel item:

| config   | req1 [100 fresh] | req2 [99 shared + 1 novel] | req2 delta |
|----------|-----------------:|---------------------------:|-----------:|
| baseline |         5966 ms  |                    5543 ms |    -423 ms |
| granular |         5962 ms  |                    5136 ms |    -826 ms |

Direct req2 comparison: granular is 406 ms faster than the
all-or-nothing baseline, matching the encoder cost of 99 Pixtral +
projector passes that granular skips.

Signed-off-by: Aswin Visva <31215515+aswinvisva@users.noreply.github.com>

… slicer

* Why?

To land ahead of the item-level MM encoder scheduling work on
yechank-nvidia/multimodal-encoder-runtime-scheduling without forcing a
churny rebase, the partial-cache primitives need a stable public
surface his executor can call into. Two smaller shifts fall out
naturally:
  - the per-model slicing hook can be a mixin default for the two
    common layouts, removing the only Mistral 3 code delta;
  - the second opt-in flag is redundant with `supports_encoder_cache`.

* What?

- Rename the partial-cache primitives to drop the underscore prefix so
  callers outside the mixin can consume them:
    `_partition_encoder_cache`   -> `partition_encoder_cache`
    `_assemble_full_embedding`   -> `assemble_full_embedding`
    `slice_multimodal_data_items` -> `build_multimodal_encoder_input`
  The last rename also aligns naming with the item-scheduling branch,
  which already uses `build_multimodal_encoder_input`.

- Provide a default `build_multimodal_encoder_input` in the mixin
  covering the two common single-modality layouts:
    * Stacked on dim 0 (Pixtral / LLaVA-family): slice `pixel_values`
      dim 0 and the parallel `image_sizes` list.
    * Packed with `*_grid_thw` offsets (Qwen2-VL family): prefix-sum
      patch counts to locate each item's slab, concat requested items
      in item-index order, slice `image_grid_thw` in parallel.
  Models with a different layout override.

- Drop `supports_granular_encoder_cache`. `supports_encoder_cache = True`
  already carries the opt-in signal; a model that opts into the cache
  and has an unhandled layout gets a loud `NotImplementedError` from
  the default slicer -- the correct signal to override.

- Delete Mistral 3's `build_multimodal_encoder_input` override: the
  default now covers it. Mistral 3's net diff shrinks to zero.

* Numbers

Same 1x H200 / Mistral-Small-3.1-24B / N=100 / 99-shared A/B run as
the previous commit, now with the mixin default doing the slicing:

  req1 [100 fresh]           5928 ms
  req2 [99 shared + 1 novel] 5114 ms  (delta -814 ms / -13.7%)

Within measurement noise of the override-based numbers, confirming
the default reproduces the override end-to-end.

Signed-off-by: Aswin Visva <31215515+aswinvisva@users.noreply.github.com>