JAX and PyTorch constructors disagree on identical config: same value builds on one backend, raises on the other

Following on from #92 (PyTorch constructor) — I ran the same matrix against the JAX constructor, and the two backends disagree on identical inputs, in both directions.

Both at `0`, on `b15593e4c1111ddb5f4f30dd2957df2edbaa04ca`:

| parameter | PyTorch | JAX |
|---|---|---|
| `embed_dim` | builds OK | `ZeroDivisionError` |
| `max_classes` | builds OK | `ZeroDivisionError` |
| `row_num_cls` | builds OK | `ZeroDivisionError` |
| `ff_factor` | builds OK | `ZeroDivisionError` |
| num frequencies | builds OK | `ZeroDivisionError` |
| `col_num_blocks` | builds OK | builds OK |
| `feature_group_size` | builds OK | builds OK |

And the asymmetry reverses for negatives — `feature_group_size=-1` **builds a model on JAX**, while PyTorch raises `RuntimeError: zeros: Dimension size must be non-negative`.

So the same `config.json` can produce a working model on one backend and an exception on the other, and which one is stricter depends on the parameter. Since `convert_and_upload.py` converts a JAX checkpoint to PyTorch and runs a parity check, I think the intent is that the two agree.

The `-1` failures on the JAX side are also worth a look on their own — `col_num_blocks=-1` surfaces as `MLIRError: Invalid type`, which is a long way from telling the caller that a block count cannot be negative.

None of this bites the default shapes. It matters if you consider the two backends interchangeable, because right now a configuration that is valid on one is not necessarily valid on the other.

I would guess the fix is the same shared validation suggested in #92 and #94, applied once and used by both constructors, rather than patching each backend separately — but that is your call on the architecture.

Disclosure: I used an AI assistant to help find this. I ran both matrices myself.

Compare the JAX and PyTorch constructors discussed in the issue, then inspect convert_and_upload.py and its parity check. Reproduce the listed zero and negative-value matrices against the referenced commit, and verify that both constructors consistently accept or reject each configuration with clear errors.

Written by the indexing model from the issue text.