ggml-webgpu: FlashAttention refactor + standardize quantization support (#23834)

* Start work on flash_attn refactor

* Refactor

* Split k/v quantization

* Refactor and abstract quantization logic for flash_attn and mul_mat

* Add quantization support to tile path

* formatting

* Move to functions, add a check

ggml/src/ggml-webgpu/pre_wgsl.hpp

@@ -37,15 +37,33 @@ static std::string trim(const std::string & s) {
 }
 
 static std::string trim_value(std::istream & is) {
-    std::string str;
-    std::getline(is, str);
-    return trim(str);
+    std::ostringstream ss;
+    ss << is.rdbuf();
+    return trim(ss.str());
 }
 
 static bool isIdentChar(char c) {
     return std::isalnum(static_cast<unsigned char>(c)) || c == '_';
 }
 
+static bool endsWithContinuation(const std::string & line) {
+    size_t i = line.size();
+    while (i > 0 && std::isspace((unsigned char) line[i - 1])) {
+        i--;
+    }
+    return i > 0 && line[i - 1] == '\\';
+}
+
+static void stripContinuation(std::string & line) {
+    size_t i = line.size();
+    while (i > 0 && std::isspace((unsigned char) line[i - 1])) {
+        i--;
+    }
+    if (i > 0 && line[i - 1] == '\\') {
+        line.erase(i - 1);
+    }
+}
+
 static std::string expandMacrosRecursiveInternal(const std::string &                                  line,
                                                  const std::unordered_map<std::string, std::string> & macros,
                                                  std::unordered_set<std::string> &                    visiting);
@@ -595,19 +613,31 @@ class Preprocessor {
         std::string        line;
 
         while (std::getline(in, line)) {
-            std::string t = trim(line);
+            std::string logical = line;
+            std::string t       = trim(logical);
+            if (!t.empty() && t[0] == '#') {
+                while (endsWithContinuation(logical)) {
+                    stripContinuation(logical);
+                    if (!std::getline(in, line)) {
+                        break;
+                    }
+                    logical += "\n";
+                    logical += line;
+                }
+                t = trim(logical);
+            }
 
             if (!t.empty() && t[0] == '#') {
                 bool handled = handleDirective(t, out, macros, predefined_macros, cond, include_stack, mode);
                 if (mode == DirectiveMode::IncludesOnly && !handled) {
-                    out << line << "\n";
+                    out << logical << "\n";
                 }
             } else {
                 if (mode == DirectiveMode::IncludesOnly) {
-                    out << line << "\n";
+                    out << logical << "\n";
                 } else if (condActive(cond)) {
                     // Expand macros in the line before outputting
-                    std::string expanded = expandMacrosRecursive(line, macros);
+                    std::string expanded = expandMacrosRecursive(logical, macros);
                     out << expanded << "\n";
                 }
             }