[BUG] Inconsistent representations between batch and single inference in Uni-Mol2 (Suspected Self-Attention Mask Issue)

### Describe the bug

When using uni-mol2, I observed that the molecular representation results differ when processing molecules in a batch versus processing them individually. The results are inconsistent for the same molecule, except for the molecule with the longest atom count in the batch.

Since batch inference requires padding all inputs to the maximum length, I suspect that the longer molecules force the smaller ones to extend with padding tokens, but these tokens are not being correctly masked during the self-attention mechanism.

**Debugging & Analysis:**
I debugged the code and traced the issue to _unimol_tools.models.transformersv2.py_. Specifically, inside the forward method of _class TransformerEncoderLayerV2_, during the first self-attention operation:

```
self.self_attn(
x,
x,
x,
pair=pair,
mask=self_attn_mask,
)
```

I found that the representation _x_ for the same molecule diverges after this step when comparing batch mode vs. single mode. Furthermore, during batch input, even when padding tokens are present, **the _self_attn_mask_ remains a matrix of all zeros**. This indicates that the padding tokens are not being masked out effectively.

The self_attn_mask is defined in _unimol_tools.models.unimolv2.UniMolV2Model.forward_ as:

`attn_mask = attn_bias.clone()`

However, attn_bias originates from get_graph_features in unimol_tools.data.conformer.py, where it is defined as:

`feat["attn_bias"] = np.zeros((mask.sum() + 1, mask.sum() + 1), dtype=np.float32)`

This initializes it as all zeros by default, ignoring the padding requirements for batch processing.

**Proposed Solution:**
I believe the issue lies in attn_mask = attn_bias.clone(). I modified the code to explicitly handle masking for padding tokens:

```
attn_mask = torch.cat([torch.ones(atom_mask.size(0), 1).to(atom_mask.device), atom_mask.clone()], dim=1)
attn_mask.masked_fill_(attn_mask.logical_not(), float("-inf"))
attn_mask = attn_mask.unsqueeze(1).repeat(1, attn_mask.size(1), 1)
```

After applying this fix, the inconsistency issue was resolved (remaining minor numerical differences appear related to _op_mask_ and _op_norm_ operation ?).

Thank you for looking into this.

### Uni-Mol Version

Uni-Mol2

### Expected behavior

The molecular representations obtained during batch inference should be consistent with those obtained during single-sample inference. The presence of padding tokens in a batch should not affect the embeddings of other molecules.

### To Reproduce

```
from unimol_tools import UniMolRepr

clf = UniMolRepr(data_type='molecule',
remove_hs=False,
model_name='unimolv2', # avaliable: unimolv1, unimolv2
model_size='164m', # work when model_name is unimolv2. avaliable: 84m, 164m, 310m, 570m, 1.1B.
)

# batch smiles unimol representation
l = ['C(CC(O)(P(=O)(O)O)P(=O)(O)[O-])CN.O.O.O.[Na+]','c1ccc(cc1)C2=NCC(=O)Nc3c2cc(cc3)[N+](=O)[O]']
r = clf.get_repr(l)
print(r[0][:4])

#single smiles unimol representation
l = ['C(CC(O)(P(=O)(O)O)P(=O)(O)[O-])CN.O.O.O.[Na+]']
r = clf.get_repr(l)
print(r[0][:4])
```

### The output before fixing the mask problem

> [ -9.299404 15.081673 -25.31543 -11.924848]
> [ -7.4061403 17.232798 -15.8836975 -4.884043 ]

### The output after fixing the mask problem
> [ -7.503006 17.204712 -15.879755 -4.7829766]
> [ -7.4061403 17.232798 -15.883697 -4.884042 ]

## fixed content: change "attn_mask = attn_bias.clone()" to:
```
attn_mask = torch.cat([torch.ones(atom_mask.size(0), 1).to(atom_mask.device), atom_mask.clone()],dim=1)
attn_mask.masked_fill_(attn_mask.logical_not(), float("-inf"))
attn_mask = attn_mask.unsqueeze(1).repeat(1, attn_mask.size(1),1)
```

### Environment

Ubuntu 24.04.2 LTS

### Additional Context

While this fixes the inference logic, I am unsure if the released pre-trained models were trained with this bug present (i.e., trained in batches without proper masking). If so, this fix might affect compatibility with the pre-trained weights.