[BUILD] Use fmt-header instead of libfmt (#1791)

src/include/proxy_trace/proxy_trace.h

@@ -10,6 +10,9 @@
 #include <chrono>
 #include <cstdint>
 #include <deque>
+#ifndef FMT_HEADER_ONLY
+#define FMT_HEADER_ONLY 1
+#endif
 #include <fmt/format.h>
 #include <memory>
 #include <unordered_map>

src/misc/proxy_trace/proxy_trace.cc

@@ -9,7 +9,6 @@
 #include "debug.h"
 #include "device.h"
 #include "proxy.h"
-#include <fmt/format.h>
 #include <map>
 
 constexpr int32_t kFinishedProxyOpItems = 32;
@@ -261,4 +260,4 @@ void facebook_rccl::addNewProxyOp(std::unique_ptr<ProxyTrace> &proxyTraceObj,
     proxyTraceObj->addNewProxyTraceOpImpl(key, extraInfo, opType, channelId,
                                           nSteps, nbytes, peerRank);
   }
-}
+}