Bidirectional RNN Concatenation

Problem Statement

Combine forward and backward hidden states from a Bidirectional RNN.

Background

In Bidirectional RNNs, we process the sequence in both directions:

• Forward pass: left to right
• Backward pass: right to left

The final output at each timestep is typically the concatenation of both directions' hidden states, doubling the feature dimension.

Your Task

Write a function combine_bidirectional(forward_states, backward_states) that concatenates the forward and backward hidden states along the last dimension.

Input Format

• forward_states: numpy array of shape (batch_size, seq_len, hidden_dim)
• backward_states: numpy array of shape (batch_size, seq_len, hidden_dim)

Output Format

Return a numpy array of shape (batch_size, seq_len, 2 * hidden_dim).