Add standalone usage example and inference-only note

[haix716]

Problem

The README only shows integration via FLA's chunk_kda interface. There is no standalone example showing direct usage of the flash_kda.fwd API, and no minimal reproduction script.

Recommendation

Add examples/basic_usage.py:

import torch
from flash_kda import fwd

B, H, T, D = 2, 8, 1024, 128
q = torch.randn(B, H, T, D, dtype=torch.bfloat16, device='cuda')
k = torch.randn_like(q)
v = torch.randn_like(q)
g = torch.randn(B, H, T, dtype=torch.float32, device='cuda')
beta = torch.rand(B, H, T, dtype=torch.float32, device='cuda').sigmoid()
scale = D ** -0.5

out = fwd(q, k, v, g, beta, scale)
print(f"Output shape: {out.shape}")  # [2, 8, 1024, 128]

Also add a note in README that this is inference-only (no backward pass) — users needing training should fall back to FLA's Triton path.

Impact

Documentation. Reduces friction for new users who want to try the kernel directly without understanding FLA's internals.