Regression: `layer_norm()`/`rms_norm()` always fails to find weight/bias tensors [main branch]

Introduced by commit b96522e6f2e59af87c12ea66e77417c195044dd9

"fixes for BERT safetensors: weight/bias gamma/beta compatibility & BERT variable prefix (#1888)"

Before: weight/bias tensors were fetched directly with vb.get_with_hints(...). Bias was only fetched when config.affine was true.

After: a name is picked first via vb.contains_tensor(name), checking ["weight", "gamma"] and ["bias", "beta"]. Missing tensor is a hard error ("Failed to find weight tensor").

The bias-name lookup is unconditional. It runs even when config.affine is false. It also runs before the real get_with_hints call for both weight and bias.

Bug 1: RmsNorm always fails

rms_norm() calls layer_norm() with affine: false. RmsNorm has no bias tensor. The unconditional bias-name lookup finds neither bias nor beta. Every rms_norm() call errors with "Failed to find weight tensor".

This breaks nearly every RmsNorm-based model. RmsNorm has no bias by definition.

Bug 2: contains_tensor breaks lazy/VarMap construction

contains_tensor(name) returns false for a tensor a VarMap-backed VarBuilder has not yet materialized, even when it can be lazily created via Init. So even the weight lookup fails for a fresh VarMap, regardless of affine.

This breaks any model built from VarBuilder::from_varmap with a fresh VarMap (e.g. tests, random init), for both affine and non-affine configs.

Reproducer

use candle_core::{DType, Device};
use candle_nn::{VarBuilder, VarMap};

let device = Device::Cpu;
let varmap = VarMap::new();
let vb = VarBuilder::from_varmap(&varmap, DType::F32, &device);

// Fails: "Failed to find weight tensor"
let _ = candle_nn::rms_norm(8, 1e-6, vb.pp("norm"));

Gate the bias-name lookup behind config.affine, matching the pre-#1888 behavior. Keep the weight/gamma and bias/beta aliasing for BERT compat, but only probe for tensor presence when actually needed, and don't let contains_tensor block legitimate lazy Init-based creation.

@Christof23