test: add GPU-free unit tests for host-side utility modules

[Dashener2]

Problem

Importing deep_ep runs check_nccl_so() and init_jit(), which require the compiled
extension and a NCCL install, so the pure-Python helpers under deep_ep/utils/ could not
be tested on a machine without a GPU. While adding those tests, hash_tensor() turned out
to raise for common inputs.

Root cause

deep_ep/utils/math.py:

def hash_tensor(t: torch.Tensor) -> int:
    return t.view(torch.int).sum().item()

view(torch.int) requires a contiguous tensor with a 4-byte element size. It raises for
bool, int64, float16/bfloat16, and for non-contiguous tensors. hash_tensors is a
public helper under deep_ep.utils.math, so any caller hashing non-float32 test data hits
this.

Fix

Load the host modules directly by path so the tests do not import deep_ep, and make
hash_tensor dtype/layout agnostic:

def hash_tensor(t: torch.Tensor) -> int:
    t = t.contiguous()
    if t.element_size() == 4:
        return t.view(torch.int).sum().item()
    return t.view(torch.uint8).sum().item()

Testing

No GPU, no CUDA and no compiled extension required:

python tests/utils/test_math.py
python tests/utils/test_semantic.py
python tests/utils/test_testing.py

Verified with torch 2.8.0+cpu and numpy 1.26. The added hash test fails against the old
implementation with RuntimeError: self.stride(-1) must be 1 to view Long as Int.