Delete FlexAttention + NJT composition from tutorial (#3561)

Fix broken CI
https://github.com/pytorch/tutorials/actions/runs/17684562516/job/50266336429?pr=3553#step:9:11267

We should delete this because NJT support will be deleted from
FlexAttention in pytorch/pytorch#161734

Co-authored-by: Svetlana Karslioglu <[email protected]>

Author: mikaylagawarecki

intermediate_source/transformer_building_blocks.py

@@ -564,7 +564,6 @@ def benchmark(func, *args, **kwargs):
 #
 # * Cross Attention
 # * Fully masked rows no longer cause NaNs
-# * Modifying attention score: ALiBi with FlexAttention and NJT
 # * Packed Projection
 
 ###############################################################################
@@ -668,66 +667,6 @@ def benchmark(func, *args, **kwargs):
 # appropriately makes it possible to properly express empty sequences.
 
 
-################################################################################
-# FlexAttention + NJT
-# ---------------------------------------------------------------------
-# NJT also composes with the ``FlexAttention`` module. This is a generalization
-# of the ``MultiheadAttention`` layer that allows for arbitrary modifications
-# to the attention score. The example below takes the ``alibi_mod``
-# that implements `ALiBi <https://arxiv.org/abs/2108.12409>`_ from
-# `attention gym <https://github.com/meta-pytorch/attention-gym>`_ and uses it
-# with nested input tensors.
-
-from torch.nn.attention.flex_attention import flex_attention
-
-
-def generate_alibi_bias(H: int):
-    """Returns an alibi bias score_mod given the number of heads H
-    Args:
-        H: number of heads
-    Returns:
-        alibi_bias: alibi bias score_mod
-    """
-
-    def alibi_mod(score, b, h, q_idx, kv_idx):
-        scale = torch.exp2(-((h + 1) * 8.0 / H))
-        bias = (q_idx - kv_idx) * scale
-        return score + bias
-
-    return alibi_mod
-
-
-query, key, value, _ = gen_batch(N, E_q, E_k, E_v, device)
-n_heads, D = 8, E_q // 8
-alibi_score_mod = generate_alibi_bias(n_heads)
-query = query.unflatten(-1, [n_heads, D]).transpose(1, 2).detach().requires_grad_()
-key = key.unflatten(-1, [n_heads, D]).transpose(1, 2).detach().requires_grad_()
-value = value.unflatten(-1, [n_heads, D]).transpose(1, 2).detach().requires_grad_()
-out_flex2 = flex_attention(query, key, value, score_mod=alibi_score_mod)
-
-###############################################################################
-# In addition, one can also use the ``block_mask`` utility of ``FlexAttention``
-# with NJTs via the ``create_nested_block_mask`` function. This is useful for
-# taking advantage of the sparsity of the mask to speed up the attention computation.
-# In particular, the function creates a sparse block mask for a "stacked sequence" of all
-# the variable length sequences in the NJT combined into one, while properly masking out
-# inter-sequence attention. In the following example, we show how to create a
-# causal block mask using this utility.
-
-from torch.nn.attention.flex_attention import create_nested_block_mask
-
-
-def causal_mask(b, h, q_idx, kv_idx):
-    return q_idx >= kv_idx
-
-
-query, key, value, _ = gen_batch(N, E_q, E_k, E_v, device)
-block_mask = create_nested_block_mask(causal_mask, 1, 1, query, _compile=True)
-query = query.unflatten(-1, [n_heads, D]).transpose(1, 2).detach().requires_grad_()
-key = key.unflatten(-1, [n_heads, D]).transpose(1, 2).detach().requires_grad_()
-value = value.unflatten(-1, [n_heads, D]).transpose(1, 2).detach().requires_grad_()
-out_flex = flex_attention(query, key, value, block_mask=block_mask)
-
 ###############################################################################
 # Packed Projection
 # -----------------