[js/webgpu] Fix Tanh explosion (#19201)

### Description
```math
\tanh(x)=\frac{e^x-e^{-x}}{e^x+e^{-x}}=
\left\{
\begin{array}{cc}
-\frac{1-e^{-2\cdot(-x)}}{1+e^{-2\cdot(-x)}}, & x<0 \\
0, & x=0 \\
\frac{1-e^{-2x}}{1+e^{-2x}}, & x>0
\end{array}
\right.
```

### Motivation and Context
On some platforms,
$$\tanh(1000)=\frac{e^{1000}-e^{-1000}}{e^{1000}+e^{-1000}}$$ would
produce NaN instead of 0.999... or 1 (imagine $e^{1000}=\infty$ and
$\frac{\infty}{\infty}$ explodes).

js/web/lib/wasm/jsep/webgpu/ops/unary-op.ts

@@ -279,7 +279,9 @@ export const tan = (context: ComputeContext): void => {
 };
 
 export const tanh = (context: ComputeContext): void => {
-  context.compute(createElementwiseProgramInfo(context.inputs[0], 'Tanh', 'tanh'));
+  // TODO: revisit after https://github.com/gpuweb/gpuweb/issues/4458 is resolved
+  context.compute(createElementwiseProgramInfo(
+      context.inputs[0], 'Tanh', a => `sign(${a}) * (1 - exp(-2 * abs(${a}))) / (1 + exp(-2 * abs(${a})))`));
 };
 
 export const thresholdedRelu = (context: ComputeContext, attributes: AlphaAttributes): number => {

js/web/test/data/ops/tanh.jsonc

@@ -0,0 +1,26 @@
+[
+  {
+    "name": "tanh with no attributes",
+    "operator": "Tanh",
+    "attributes": [],
+    "cases": [
+      {
+        "name": "T[2,4]",
+        "inputs": [
+          {
+            "data": [-1000, -1, 0, 0.1, 0.2, 0.3, 0.4, 1000],
+            "dims": [2, 4],
+            "type": "float32"
+          }
+        ],
+        "outputs": [
+          {
+            "data": [-1, -0.761594, 0, 0.099668, 0.197375, 0.291313, 0.379949, 1],
+            "dims": [2, 4],
+            "type": "float32"
+          }
+        ]
+      }
+    ]
+  }
+]

js/web/test/suite-test-list.jsonc

@@ -1389,6 +1389,7 @@
       "sub.jsonc",
       "sub_int32.jsonc",
       "tan.jsonc",
+      "tanh.jsonc",
       "tile.jsonc",
       "transpose.jsonc",
       "transpose_int32_uint32.jsonc",