[FlaxWav2Vec2Model] Fix bug in attention mask
Currently, the FlaxWav2Vec2 reduced attention mask is computed by calling the function `_get_feat_extract_output_lengths`, without explicit specification of whether an (optional) adapter module is used:
https://github.com/huggingface/transformers/blob/924484ee4a6ebc79426d27eef31a1ee7d13cbb9a/src/transformers/models/wav2vec2/modeling_flax_wav2vec2.py#L959-L960
By default, if `add_adapter` is `None`, the boolean `add_adapter` will be set based on the `config`:
https://github.com/huggingface/transformers/blob/924484ee4a6ebc79426d27eef31a1ee7d13cbb9a/src/transformers/models/wav2vec2/modeling_flax_wav2vec2.py#L1001-L1008
For this default setting, if the model contains an adapter module, then `add_adapter` will be set to `True`. This results in the convolutional formula including the downsampling performed by the convolutional layers in the feature extractor **and** the adapter module.

However, since the reduced attention mask is required for the encoder module, it should be computed based on the convolutional layers of the feature extractor **only**, and not those of the subsequent adapter module. This is highlighted by the PyTorch Wav2Vec2 modelling code:
https://github.com/huggingface/transformers/blob/924484ee4a6ebc79426d27eef31a1ee7d13cbb9a/src/transformers/models/wav2vec2/modeling_wav2vec2.py#L1350-L1354

The following code snippet demonstrates the effect of this bug by means of a PyTorch-Flax cross-test:
```python
import torch
import numpy as np
from transformers import Wav2Vec2Model, FlaxWav2Vec2Model
import tempfile
import random

encoder_id = "hf-internal-testing/tiny-random-wav2vec2"

fx_model = FlaxWav2Vec2Model.from_pretrained(encoder_id, add_adapter=True, from_pt=True)

with tempfile.TemporaryDirectory() as tmpdirname:
    fx_model.save_pretrained(tmpdirname)
    pt_model = Wav2Vec2Model.from_pretrained(tmpdirname, config=fx_model.config, from_flax=True)


# create synthetic input data
def ids_tensor(shape, vocab_size, rng=None):
    """Creates a random int32 tensor of the shape within the vocab size."""
    if rng is None:
        rng = random.Random()

    total_dims = 1
    for dim in shape:
        total_dims *= dim

    values = []
    for _ in range(total_dims):
        values.append(rng.randint(0, vocab_size - 1))

    output = np.array(values).reshape(shape)

    return output


def random_attention_mask(shape, rng=None):
    attn_mask = ids_tensor(shape, vocab_size=2, rng=rng)
    # make sure that at least one token is attended to for each batch
    attn_mask[:, -1] = 1
    return attn_mask


def floats_tensor(shape, scale=1.0):
    """Creates a random float32 tensor"""
    total_dims = 1
    for dim in shape:
        total_dims *= dim

    values = []
    for _ in range(total_dims):
        values.append(np.random.randn() * scale)

    return np.array(values, dtype=np.float32).reshape(shape)


def fx_batch(batch_size=2, input_length=96000):
    input_ids = floats_tensor([batch_size, input_length])
    attention_mask = random_attention_mask([batch_size, input_length])
    
    fx_inputs = {
        "input_values": input_ids,
        "attention_mask": attention_mask,
    }
    return fx_inputs


fx_inputs = fx_batch()
pt_inputs = {k: torch.tensor(v.tolist()) for k, v in fx_inputs.items()}

fx_outputs = fx_model( **fx_inputs, output_hidden_states=True)
pt_outputs = pt_model(**pt_inputs, output_hidden_states=True)

# helper function for our analysis
def assert_almost_equals(a: np.ndarray, b: np.ndarray, tol: float = 1e-2):
    diff = np.abs((a - b)).max()
    if diff < tol:
        print(f"✅ Difference between Flax and PyTorch is {diff} (< {tol})")
    else:
        print(f"❌ Difference between Flax and PyTorch is {diff} (>= {tol})")


print("--------------------------Checking hidden states match--------------------------")
for fx_state, pt_state in zip(fx_outputs.hidden_states, pt_outputs.hidden_states):
    assert fx_state.shape == pt_state.shape
    assert_almost_equals(fx_state, pt_state.detach().numpy())


print("--------------------------Checking last hidden states match--------------------------")
print(f"Encoder-decoder output shape: {fx_outputs.last_hidden_state.shape}, encoder-only output shape: {pt_outputs.last_hidden_state.shape}")
assert_almost_equals(fx_outputs.last_hidden_state, pt_outputs.last_hidden_state.detach().numpy())
```
Output prior to fix:
```
--------------------------Checking encoder hidden states match--------------------------
❌ Difference between Flax and PyTorch is 0.43152332305908203 (>= 0.01)
❌ Difference between Flax and PyTorch is 0.43074753880500793 (>= 0.01)
❌ Difference between Flax and PyTorch is 0.42613524198532104 (>= 0.01)
❌ Difference between Flax and PyTorch is 0.4301084578037262 (>= 0.01)
❌ Difference between Flax and PyTorch is 4.519614219665527 (>= 0.01)
--------------------------Checking encoder last hidden states match--------------------------
Encoder-decoder output shape: (2, 188, 16), encoder-only output shape: torch.Size([2, 188, 16])
✅ Difference between Flax and PyTorch is 0.0015139428433030844 (< 0.01)
```

Output following fix:
```
--------------------------Checking encoder hidden states match--------------------------
✅ Difference between Flax and PyTorch is 3.9674341678619385e-07 (< 0.01)
✅ Difference between Flax and PyTorch is 4.041939973831177e-07 (< 0.01)
✅ Difference between Flax and PyTorch is 4.041939973831177e-07 (< 0.01)
✅ Difference between Flax and PyTorch is 3.948807716369629e-07 (< 0.01)
✅ Difference between Flax and PyTorch is 4.947185516357422e-06 (< 0.01)
--------------------------Checking encoder last hidden states match--------------------------
Encoder-decoder output shape: (2, 188, 16), encoder-only output shape: torch.Size([2, 188, 16])
✅ Difference between Flax and PyTorch is 1.0913936421275139e-09 (< 0.01)
```

_The documentation is not available anymore as the PR was closed or merged._