Default HiddenAct::Gelu to GeLU + tanh in favour of GeLU erf (#753)

Co-authored-by: Alvaro Bartolome <[email protected]>

Author: vrdn-23

backends/candle/src/layers/linear.rs

@@ -5,7 +5,10 @@ use serde::Deserialize;
 #[derive(Debug, Deserialize, PartialEq, Clone)]
 #[serde(rename_all = "lowercase")]
 pub enum HiddenAct {
-    #[serde(alias = "gelu_pytorch_tanh")]
+    // NOTE: `GeluErf` is excluded due to incompatibility with cuBLASLt, as only GeLU + tanh
+    // approximation is implemented due to efficiency, so GeLU is standardized to tanh approx. with
+    // slight numerical deviation from GeLU erf (neglible on inference quality)
+    #[serde(alias = "gelu_new", alias = "gelu_pytorch_tanh")]
     Gelu,
     Relu,
     Silu,

backends/candle/src/lib.rs

@@ -180,7 +180,19 @@ impl CandleBackend {
         let config: String = std::fs::read_to_string(model_path.join("config.json"))
             .context("Unable to read config file")
             .map_err(|err| BackendError::Start(format!("{err:?}")))?;
-        let config: Config = serde_json::from_str(&config)
+
+        let config_value: serde_json::Value = serde_json::from_str(&config)
+            .context("Unable to parse config.json")
+            .map_err(|err| BackendError::Start(format!("{err:?}")))?;
+
+        if let Some(hidden_act) = config_value.get("hidden_act").and_then(|v| v.as_str()) {
+            if hidden_act == "gelu" {
+                // NOTE: https://github.com/huggingface/text-embeddings-inference/pull/753
+                tracing::warn!("The `config.json` contains `hidden_act=gelu` and GeLU + tanh approximation will be used instead of exact GeLU (aka. GeLU erf), which might lead to subtle differences with Transformers or Sentence Transformers outputs which use exact GeLU when `hidden_act=gelu`, unless specified otherwise. GeLU + tanh is more efficient and more consistent across devices (e.g., cuBLASLt comes with fused GeLU + tanh), and will have negligible impact on the inference quality.");
+            }
+        }
+
+        let config: Config = serde_json::from_value(config_value)
             .context("Model is not supported")
             .map_err(|err| BackendError::Start(format!("{err:?}")))?;
 

backends/candle/tests/snapshots/test_bert__emotions_batch.snap

@@ -2,87 +2,87 @@
 source: backends/candle/tests/test_bert.rs
 expression: predictions_batch
 ---
-- - -6.548559
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+  - -4.632993
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+  - -5.6739373
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-  - -5.7792044
-  - -8.657227
-  - -8.258305
-  - -6.64832
-  - -7.4060283
-  - 3.046496
+  - -7.5134573
+  - -6.8658743
+  - -7.1864815
+  - -7.128115
+  - -8.2107115
+  - -7.017146
+  - -7.132131
+  - -8.533407
+  - -6.229486
+  - -8.742311
+  - -5.7792006
+  - -8.65723
+  - -8.258308
+  - -6.648321
+  - -7.406026
+  - 3.0464942