[MRG] Fix device placement in batch Sinkhorn (solve_batch on GPU)

[tomMoral]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Motivation and context / Related issue

ot.solve_batch(M, reg=..., method="sinkhorn") crashes on GPU with

RuntimeError: Expected all tensors to be on the same device, but found at least two devices, cuda:0 and cpu!

whenever the torch default device is CPU while the inputs are on CUDA. In
bregman_projection_batch, the scaling vectors f/g were allocated with a
bare nx.ones((B, n)), so they landed on the default (CPU) device and clashed
with the GPU K on the first iteration. The log-domain and proximal solvers
were unaffected (their u/v inits already use type_as).

Reproduced and both the fix and the refactor verified on an NVIDIA V100S
(torch 2.6.0+cu118): all batch paths now run on cuda:0, and a marginal passed
on the wrong device now raises a clear assert_same_dtype_device error instead
of a deep RuntimeError.

How has this been tested (if it applies)

• New test_solve_batch_device exercises every method on each available device.
• New test_unif_shape / test_check_marginal cover the utility additions.

Description

Two commits:

1. Fix — give f/g (and the default a/b) the input backend/device via
   type_as in bregman_projection_batch.
2. Refactor — the if a is None: a = nx.ones(...) / n idiom was duplicated
   across all five batch solvers (each an opportunity to forget type_as).
   Add ot.utils.check_marginal (fills a uniform marginal on the inputs'
   backend/device when None, otherwise validates shape + dtype/device), extend
   ot.utils.unif to accept a shape tuple (normalized over the last axis, fully
   backward compatible), and route all five sites through check_marginal.

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check RELEASES.md).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.38710% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.84%. Comparing base (6372a66) to head (9bebead).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #851      +/-   ##
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- Coverage   96.85%   96.84%   -0.01%     
==========================================
  Files         128      128              
  Lines       26160    26195      +35     
==========================================
+ Hits        25337    25369      +32     
- Misses        823      826       +3     

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