Removes the retain_graph flag (#385)

* Adds global tracing flag
* Removes retain_graph in favor of is_tracer

mlx/array.cpp

@@ -6,6 +6,7 @@
 #include "mlx/ops.h"
 #include "mlx/primitives.h"
 #include "mlx/transforms.h"
+#include "mlx/transforms_impl.h"
 
 namespace mlx::core {
 
@@ -21,6 +22,12 @@ std::pair<size_t, std::vector<size_t>> cum_prod(const std::vector<int>& shape) {
   return {cum_prod, strides};
 }
 
+/** Return true if we are currently performing a function transformation in
+ * order to keep the graph when evaluating tracer arrays. */
+bool in_tracing() {
+  return detail::InTracing::in_tracing();
+}
+
 } // namespace
 
 array::array(const std::complex<float>& val, Dtype dtype /* = complex64 */)
@@ -62,8 +69,12 @@ void array::detach() {
   array_desc_->primitive = nullptr;
 }
 
-void array::eval(bool retain_graph /* = false */) {
-  mlx::core::eval({*this}, retain_graph);
+void array::eval() {
+  mlx::core::eval({*this});
+}
+
+bool array::is_tracer() const {
+  return array_desc_->is_tracer && in_tracing();
 }
 
 void array::set_data(allocator::Buffer buffer, deleter_t d) {