Sinusoidal Positional Encoding

Implement sinusoidal positional encoding from "Attention Is All You Need".

Signature: def positional_encoding(seq_len: int, d_model: int) -> np.ndarray

Return a matrix of shape (seq_len, d_model) where:

• Even indices: PE[pos, 2i] = sin(pos / 10000^(2i/d_model))
• Odd indices: PE[pos, 2i+1] = cos(pos / 10000^(2i/d_model))